I've been wandering the net this evening looking for my own information on dentures, and I happened to see something I think may be of use to you. --I have no idea whether this actually works, because I don't yet have dentures, myself.
In order to solve this problem, I've seen quite a few people say it helps to stay up as late as you possible can one night--to the point where you're actually falling asleep while sitting up. At the very last moment before finally lying down, slip the denture into your mouth. I've seen quite a few people say that after a week's worth of this practice, they've been able to wear their denture without gagging.
I hope it helps you. Good luck!