Your undergrad GPA stands where it is. However, it does give adcoms a chance to see that you have the potential to handle medical school courses, provided, of course, that you do well in the program. If it is simply a matter of weak undergrad GPA, you might be able to get in with only a year at the special master's program. I did, and many others who were in my program did as well. However, if you don't have other strong credentials, such as volunteering, extra-curriculars and high MCAT scores, you might need to go through the entire program for medical schools to give you serious consideration. If you apply during your first year at the master's, the med schools are really looking at only the first semester's grades, not the whole year's. As such, it is very important to have other strong credentials under your belt.
with regards