You might just be sensitive to certain foods and drinks. Is there anything in particular that you consume that means you notice you have a weak stomach? Maybe keep a food diary and see if there is anything that is particularly bad for making you notice it, and then cut down on it as your body obviosuly doesn't like it.
With your gag reflex, it is just natural really. You could try and train it with something like a banana and putting more in your mouth that usual before you bit it off. Stop when you start to gag and try again until you can get it further back more comfortably.
If you still have problems then go and see a doctor as you might have a digestive disorder or something similar that is making you this way so it is best to go and have a chat about it to try and find out and help you in the long run :)
Wanna know a random trick? If you make a fist with your left hand and squeeze your left thumb inside of it, you won't have a gag reflex.
I'm sensitive to too much cheese :(
I actually already knew that!! :)