Personally, I don't think Gemma has mass appeal outside of the fashion world. Gisele's face doesn't really have the mass appeal but her body does, which Gemma also doesn't have (but she could develop it?).
Additionally, Gisele has great personality and drive. In interviews, she is always having a blast, laughing, etc. She does what it takes to get noticed - going to parties/award shows, making appearances in movies/tv, dating someone famous, appearing on talk shows. Gemma, on the other hand, doesn't seem as if she wants all that and she just seems a little shy/boring backstage, although she is very cute.