After a meal I will sometimes feel nauseous and gag as well I wear uper dentures and this stops after I remove them what could cause this?

Sensitive Gag Reflex. Under stress and anxiety it’s very common for a body system or specific symptom to surface. This is a fairly common symptom and is not related to a significant health or mental health problem. Ask your dentist for desensitization Sensitive Gag Reflex exercise.
More info needed... How long have you had the dentures? Are they new? Have you always gagged when eating with this full denture? They may be ill fitting and moving without you realizing, but most likely they are overextended into your soft palate. Please have a local dentist take a look to evaluate the fit. No use gagging when you eat.