Gust of Wind's success result is "The creature can't move against the wind." It doesn't carry over to failure or critical failure. So if cast in a 5-ft wide corridor, it's vastly superior if the enemy succeeds, no? They can't move towards you at all during their turn.
There are maybe some cases that isn't true, but I even thought about the critical failure result. It's a 30-foot push back + prone, but even that is less restricting than the success result for most creatures.
Is this a correct reading on it? It's a curious interaction for a very specific grid layout.
Well, you could rule that creatures that attempt to enter the spells area again suffer the consequences of the spell again using the same saving throw result as they got the first time.
Some spells like Circle of Protection specifically say that repeated attempts against a spell don't get rolled again - you just use the result of the first attempt.
Other spells, like Bane don't specifically say that, but it is somewhat implied since otherwise there exist shenanigans that opponents can use to try and fish for better save results.
Oh wait, you made me notice something. The spell has a duration. I've only attributed that to the success result and the opportunity for creatures entering later on. Everything else about the spell seems to be an instant, one-time effect.
Could it be the duration is also applied to the prone condition? It would make the failure results unanimously superior. I have my doubts, since I expect it would point out being unable to end prone by standing. It's also a level 1 spell.
In the end, it might just be, like you said, re-entering re-triggers the spell's conequences.
This is more of a ruling in the face of ambiguous and vague rules rather than RAW. That said, I would not prevent them from standing up during the duration of the spell, and if they choose to stay prone then I wouldn't apply the spell effect to them again even if they are in the area. For the failure effect this is rather equivalent to simply forbidding them from standing up. For the critical failure, if they stay prone, then they wouldn't end up being moved and take damage again. In either case they could crawl out of the spell's area of effect though.
So yeah, it makes decent crowd control for a level 1 spell - as long as you are fighting in a narrow corridor. Otherwise, bypassing it is reasonably easy and it just ends up taking actions from the enemies and only on saving throw failure.