When we do hypotheticals like this, why are we usually making the larger 'named' fighter come down in weight rather than the smaller fighter seeking glory climb up in weight like the fighters of yesterday? GGG punches a few dents into Floyd's chest at 160 lbs. I see no genuine reason to suppose him at 154 lbs, when he was a 160 lbs champion, for any fighter/fight.
GGG fought his last several years in the amateurs at 165 lbs weight division. To be honest, i dont think Golovkin would ever fight Floyd at 154lbs. It would likely be at 157lbs catch weight at which case GGG would look like he did in the Brook fight. When GGG said he would fight anybody from 154lbs to 168lbs i dont think he ever meant he was going down to 154lbs. I'm sure he had Floyd in mind and thats about it. He would come down at an agreed upon weight for Floyd only. Floyd was the real big money. For Canelo,GGG would never come down because Canelo was too big boned. Canelo was assentially already a middleweight fighting at154lbs. My gut says at a catch weight GGG stops him late, similar to the Brook fight and the Ouma fight. The savageness of GGG would overwhelm Floyd late, around rounds 10 through 12. Floyd doesnt have the power to keep GGG off him, but you never know, Floyd is a boxing genius, that why it would have been a super fight. No way GGG would go down to 154lbs, at least he shouldn't.
Close decision unless GGG gasses out from the weight draining,it could've went either way because of the size advantage too.
Let's talk about two fighters who fought in different weight classes. Mayweather has never fought in the middleweight and has only fought 3 times in the superwelterweight (including the one with McGregor....), GGG has always fought in the middleweight (except for the last match in his career against Canelo....). If I have to think of GGG at the limit of 154 pounds I think he would be weakened and therefore Floyd would win on points. But it would be more logical for Pretty Boy to move up to the middleweight category and in that case I say GGG by tko in the last rounds. Mayweather was a great champion and a master of defense but he has never faced a pure middleweight with the power of GGG and his ability to cut the ring.
This has nothing to do with a Mayweather vs. GGG fantasy match at middleweight. GGG has dominated middleweight for years and Floyd has never fought at middleweight and has never met a middleweight with GGG's characteristics (Cotto, Alvarez and De La Hoya did not have them), simple as that.