I am 25, 5'11 and my bmi is only 16. i suffer from gastroparesis is there anything i can do to gain weight? at what bmi should i be worried?

BMI 16: Yes you can and must gain weight! Start by seeing your gastroenterologist and do so as soon as possible.

BMI 16 is pretty low: Gastroparesis causes may be mechanical (obstruction, or the stomach outlet narrowed from healing of past inflammation, treated by endoscopic dilation), post-inflammatory (due to swelling of the stomach lining from ulcers, treated with acid blockers), or neurologic (as with diabetes, treated with rx that stimulates motility--domperidone, erythromycin, reglan, (metoclopramide) zelnorm) & electric stomach pacemaker.

Should worry now.: That BMI suggests significant malnutrition, which should worry you now. You really need a definitive diagnostic and management plan for your nutrition.