I don't think a "w" affects your GPA, but more importantly it's something that an adcom could give you sh1t about:
"So, swifteagle43, I see you have a withdrawal on your transcript. Why'd you give up?"
There's really no correct answer you could give unless you had to deal with a death in the family at the time or something. All I'm saying is that you'll deal with a TON of B.S. in med school, and if I was on an adcom and in a bad mood that day, I'd probably grill you over how much you could handle as a med student.
I'm not being a d1ck, just trying to be honest. If you withdraw and it appears on your official transcript, you'd better have a pretty damn good reason.
NS