Nope, it's not necessarily just based on GPA. I'm sure some people were accepted into vet prep because they had a lower than average GPA, but I know someone in the program that is 33 years old, has worked in an ER clinic for 10 years, but hasn't been in school for 8 years. So, Ross wanted to make sure she had the chance to get back into education smoothly.
Another person in Vet prep had an average/above average GPA, but had minimal clinical experience. So, Ross wanted to make sure she was able to get some early hands-on opportunities. Seems like the Vet prep students get to do a lot more hands-on stuff (makes me jealous).