What causes weight gain, nausea and fullness in the abdomen?

Maybe not related? First distinguish fluid weight (like edema, ascites, "third spacing") from tissue weight (e.g. Bodybuilding, reparative, tumors, adipose). There are lots of metabolic reasons to rule out (organ disease like heart, liver, kidneys), insufficiency (thyroid, adrenal, kidney), more. Often high calorie (but low volume) foods & drink may add up quickly (3000-3500 extra calories per pound gained).