Where did you find that? I'm not aware of such a term.
Reading it, a coverage gap is probably a place between AP's where you do not have coverage from any AP, for example if you cross floors with lifts/stairs resulting in a temporary loss of signal.
From that, a logical explanation might be that a coverage gap timeout could be the time that a client can spend in such a gap before the user/association is purged from the wireless system.
Again, I'm not aware of an official definition of 'coverage gap timeout', and to be honest it was the first time I saw it in your post. Giving some context may help...