Forgot to drink gastrografin before ct scan, what to do?

Tell the CT tech: tell the CT tech before they scan you and they can give you some to drink before the scan.

Probably okay: Policies for different imaging facilities and departments differ, but usually CTs of the abdomen and pelvis without oral contrast are fine for interpretation. Where I work, sometimes we won't even request that oral contrast be given unless it's for very specific indications.