i can understand your anxiety
but at this age common cause for stomache pain will be worm infestation also ..
has given relief to your child , then just try to modify her diet with less spices and more of bland diet and also more fruits and liquids to avoid further gastritis
in future,,make sure she eats at regular timings .. and give your child one wormectal ie albendazole
400 mg tab to combat worm infestation too..
and coming to gall bladder
problems ..of course it has family predisposition but dont get tensed and modifying the diet also reduces the chances of gall bladder problem to great extent ..
at this age probability is very less unless your diet is full of spices and oily food,,
so take good diet and avoid the problem..all the best ..