It may be different with Female Gem/Cancer Male, than Female Cancer/Gem Male... Male cancers irk my damn nerves because they're too emotional and I don't like it. But if the chick is a female, as long as she doesn't jump into the sea of emotions, the Gem should be okay. Gem's tread on the surface of a lot of things. For me, it takes a while for me to dive deeper into anything. I have to be REALLY interested...
But just go for it. The worst mistake you can make is try to generalize the guy and then live according to how you THINK he is. Be you!
So far we are different but we manage relationships the same.