I totally understand. I have a VERY sensitive gag reflex also. I have blown a barium "shake" all over an exam room! I have had 2 colonoscopies and could not drink the stuff at all.
Does your doctor have an on call number? If so, CALL IT! Let them know you are having problems and ask for an alternative. You could also call the pharmacy, but the Dr. is your best bet. If you can't reach your Dr. or his sub, then call the local ER and see if you can talk to a nurse.
I ended up having to take a bunch of OTC laxatives. I had cramping and stomach pain, but I really preferred that to gagging down and then puking up the liquid stuff. Just get some pillows, a heating pad, and reading materials and camp out in the bathroom when they start working. You may be up all night.
But the test is important. I would rather endure a day of stomach cramps and diarrhea than untreated Colon Cancer.
Good Luck