gsp isn't stupid, he would understand the rules and adjust accordingly. He's a master strategist with an aggressive fighting style who rarely fights of his back. He would of been a terror and completely destroy fighters in pride at his own weight division. 
Ansari -This would have happened to every ufc guy he fought
Think he would have done incredibly well. Watch him pre serra fight he would bust guys up on the ground even without the elbows I think he would be effective. The scrambles would also allow GSP opportunity to use knees to the head of guys he put down. GSP could cut through peoples guards easily and with a more open style I could see the opening for him to knee to the head with ease.
Additionally he was not a cage wrestler, GSP got TD's right smack in the middle of the cage unlike many other fighters who utilize the cage for it.
GSP with knees on the ground? Would have been known as the goat. 
Kelvin Gastelum fan - He still would be a point fighter, everyone saying he'd be successful because of the rules, to knee a ground opponent you have to risk giving up position. Its not like wrestlers were hugely successful. Guys like Wand and Shogun seem to have benefitted the best, not exactly wrestlers. Truth is, downed knees are most beneficial to sprawl n brawlers, not wrestlersExcept when GSP shot on you in your prime you were going on your back. There was no sprawling. He had some of the best guard passes ever. He could make it a gameplan to blast a double. Pass to side then knee to the head from there and north south.
He would have been TINY compared to the guys he was fighting. Pride's WW division had an 83 kilo (183 pounds) weight limit. He would have smashed the Japanese guys (who were tiny themselves), but the bigger gaijin would have given him problems. Guys like Hendo, Bustamante, Filho, Kang, etc.
Kelvin Gastelum fan - He still would be a point fighter, everyone saying he'd be successful because of the rules, to knee a ground opponent you have to risk giving up position. Its not like wrestlers were hugely successful. Guys like Wand and Shogun seem to have benefitted the best, not exactly wrestlers. Truth is, downed knees are most beneficial to sprawl n brawlers, not wrestlers
Well Coleman was the GP champ... the guys I remember using knees best were Coleman, Randleman, and BJJers with strong wrestling like Arona and Sperry. I don't remember any of them being kneed into a living death either.
Wand, Shogun, Ninja were more into kicking and stomping guys than kneeing them.
GSP Vs Wandy 185 in PRIDE, Saitama Super dome woulda come down.
"Yellow cards"
I can't think of many or any times that GSP would have gotten one. Certainly fewer than Arona, and I'm not aware of any decisions Arona lost except the rematch with Wand, where Arona spent more of the fight on his back with Wand standing and kicking his legs.
"moving up in weight"
Maybe.
" no elbows"
No elbows, but now he can knee from side control, and iirc GSP has passed the guard of everyone he's ever taken down.
" no cage, no wall and stall"
Non-issue, just like Arona, Sherk, Coleman, etc did not have any difficulty taking guys down without a cage, and like ho Barnett/Crocop II managed to spend half the fight pressed against the ropes. Anyway, the only time EVER I can remember GSP doing anything that might pass for "wall and stall" would be early in the second BJ fight.
" average stand up skills early in his career all would have put more losses on his records."
Maybe, although even then I don't remember anyone outstriking him.
"He would have been TINY compared to the guys he was fighting. Pride's WW division had an 83 kilo (183 pounds) weight limit. He would have smashed the Japanese guys (who were tiny themselves), but the bigger gaijin would have given him problems. Guys like Hendo, Bustamante, Filho, Kang, etc."
Hendo weighed in the 190s for most of his Pride career, barely squeaking into 200 for some of his 205 fights. Same likely goes for Busta and probably Filho.
GSP weighed 194 against Shields, he'd have been bigger in Pride with their weight classes and unrestricted juicing.
Steve4192 - He would have been TINY compared to the guys he was fighting. Pride's WW division had an 83 kilo (183 pounds) weight limit. He would have smashed the Japanese guys (who were tiny themselves), but the bigger gaijin would have given him problems. Guys like Hendo, Bustamante, Filho, Kang, etc.I feel like he'd account for it and add mass.
He would have done what GSP does and beaten everybody put in front of him.
orcus - "he could potentially lose a lot of fights by decision due to being deemed not doing enough to finish the fight since the judging criteria was different."
Which of GSP's opponents do you think have done more to attempt to finish the fight than he has?
How many sub attempts from their backs did BJ, Diaz, Condit, Serra, Fitch, and Alves have, combined? How many devastating strikes did they and Koscheck and Shields etc land? Which of them could have won a decision in Pride because they did more to finish and did more damage than he did?
The guy on top isn't losing a fight in Pride unless he's doing jack shit while the guy on the bottom is going for stuff. How many decisions did Arona lose?
"he would also be very beatable."
I can't think of anyone near his weight who could have beaten him.
This.
I really dont think it matters, ring or cage. Elbows or knees on the ground. GSP was simply too good for everyone no matter the rules.
I think the Japanese fans would have liked him as well with his personality and karate background.
Kelvin Gastelum fan - He still would be a point fighter, everyone saying he'd be successful because of the rules, to knee a ground opponent you have to risk giving up position. Its not like wrestlers were hugely successful. Guys like Wand and Shogun seem to have benefitted the best, not exactly wrestlers. Truth is, downed knees are most beneficial to sprawl n brawlers, not wrestlers
Yeah, he never passed in the UFC. Never had side control. Coleman never won the GP. Hendo never won the the MW belt. Arona was a big fail. Oh, that Saku something guy wasnt really a factor over there either.....
Later
Aside from TKO, you only need to see the Matt Serra body knees TKO to realize how huge a factor they would have been for him.
Also, if he took advantage of no drug testing to its fullest extent, he would have smashed anyone comparable in weight.
Adding in soccer kicks and stomps changed a lot.
Fuck sakes. GSP with skyscraper knees and soccer kicks would have been unreal.
Leghound - GSP with knees on the ground would be a killerThis. I think he would dominate and use the rules to his advantage. Would probably benefit him and get more finishes
orcus - "Yellow cards"
I can't think of many or any times that GSP would have gotten one. Certainly fewer than Arona, and I'm not aware of any decisions Arona lost except the rematch with Wand, where Arona spent more of the fight on his back with Wand standing and kicking his legs.
"moving up in weight"
Maybe.
" no elbows"
No elbows, but now he can knee from side control, and iirc GSP has passed the guard of everyone he's ever taken down.
" no cage, no wall and stall"
Non-issue, just like Arona, Sherk, Coleman, etc did not have any difficulty taking guys down without a cage, and like ho Barnett/Crocop II managed to spend half the fight pressed against the ropes. Anyway, the only time EVER I can remember GSP doing anything that might pass for "wall and stall" would be early in the second BJ fight.
" average stand up skills early in his career all would have put more losses on his records."
Maybe, although even then I don't remember anyone outstriking him.
GSP would have certainly got one in the BJ penn fight. IIRC he may have gotten one in the karo fight as well.
The no elbows rule combined with yellow cards would have been bad for GSP's top guard game. He would have had to posture up and strike opening himself up to triangles and armbars by very active BJJ guard players. It's either strike and get possibly submitted, or take a yellow card. It is very possible that nick diaz would have done a lot better and more competitive had the fight taken place in Pride.
I've watched all of GSP's fights and I can't recall a time when he stood in the pocket and exchanged besides the serra and hendricks fight. We all know what happened there. It was always one strike, then go back to the shooting range.
Like I said before, he would be great not unstoppable. I definitely don't see him beating Anderson, Filho, Henderson, and Prime Saku in a Pride Ring with Pride rules.