How far back does the plate go? Sometimes the dentist can trim it so that it doesn't go so far back in your mouth.
Much of gagging, however, is psychological. There's no magic fix, but I gagged when first using a full upper denture. I found that drinking water and nibbling on food while I had it in helped. Take it out when it becomes unbearable. But then try again.
Worst comes to worst and you can't get over it by yourself, there's therapy options for overcoming gagging, like hypnotherapy and exposure therapy. I've heard many success stories with these. So don't panic if it remains a problem! You've always got options. Don't be afraid to explain the problems to your dentist.