I feel like if she had any plans of coming back she would let people know. To me, it's as obvious as it can be without an official proclamation that she is done with modeling. I bet she still wants to act, though. But remember, it's very common for models to retire after a short burst of glory in the industry...it's rarer for them to hold on for years and years, especially considering they get into the business SO young and it consumes their youth.
Gemma reminds me of an Olympic medalist gymnast. Ladies like Carly Patterson, who work very hard at a young age and win a gold medal in their teens/early 20s, don't keep going to the competitions for years after that. They retire and enjoy the rest of their life.