Yes I'd say that Gemini takes most of her faces from Maurtia Falls, some maybe from the internet and various media. She has no compunction about leaving a trail of bodies, although she does use different methods to disguise her involvement but there would be a specific pattern to be found, maybe with help from others that Gemini's tangled with?

As for her body, absolutely yesss! But first of all I imagine John will try to take her down with a silver bullet to the heart SPN-style, right? NOT GONNA WORK JOHN :P

So perhaps his first attempt could go disastrously wrong (maybe even with her killing him)? So for a second attempt if he wants her captured then he's going to have to freeze (or overheat) her somehow, but he's pretty resourceful so I'm sure he can manage!

So, yes she might be a bit of a challenge, but a fun one hopefully.