Flavor the failure to a success then: in an impossible feat of strength, you managed to lift the iron gate high enough for a small creature to crawl through.
Then, depending on how important this situation is for progressing the narrative (whether you as the DM want them to get through or not), you have options to build on their success. Do they have a small creature in their party? Will they be willing to split the party? Do they have the strength to continue holding it up or is there a risk someone would be crushed crawling under? Or maybe their crit success was enough to move it but not enough to hold it at all for a chance for any one to get through.
Semantics I guess. I see your point and don't see a problem with a DM running things that way, but I don't think there's a problem with letting the players enjoy the idea of a critical success on their roll while keeping things functionally the same so as not to break the game.