Regardless of the somewhat out of place comparison to Alex, it's a fair question. Gina was being chased around the facility for a while and then finally had enough time to make a dramatic exit in front of Claire and Moira, so she had plenty of time for the virus to kick in. If she's anything like Rachel then she was definitely scared when the afflicted was chasing her, plus you could hear her scream, so I'm not sure why she didn't mutate, honestly. It was probably just too early in the story to introduce the concept, but they still had to get the "kill a useless Foley at the beginning of a revelations game" out of their system.