The average dog is a nicer person than the average person
If Gia had insecurities about herself, getting together with athletes is probably not a good idea. Dating celebrity/sports/public figures can be tough on anybody, even those with a good measure of self-esteem. Eventually someone is going to crack. Even if her boyfriend was 100% faithful, if she didn't feel worthy of his love nothing could have convinced her otherwise. From all accounts she didn't drink much so it might not have been a childhood issue. IDK, something in her psyche changed her life path, with devastating results.
Perhaps the world was just too cruel a place for someone who seemed so naive and sensitive.
Sheldon Cooper: Woman, you're playing with forces beyond your ken
Penny: Yeah, well your Ken can kiss my Barbie.