The way mine worked was that GPA was taken into account in where you were originally placed on the alternate list. And, as long as your GPA does not go down while on the list then you keep your spot. If there are enough "acceptees" that decline to come then you get tapped for an admission.
For example : Say I was placed as #10 on the alternate list and my GPA stayed the same. If there were 10 people that declined their admissions then I would be admitted. Likewise, if I got an "F" in a summer class then I could be dropped to the last spot on the list or dropped off the list all together.
Personally, the first time I applied there were 25 people on the alternate list and 18 got selected. I didn't get in so I must have been #19-25. And that was due to my GPA, which was low because of some mistakes made by my CC.