I agree with the others on this thread that said she cried because it touched a nerve. I think she knows deep down that she has made the wrong move by siding with GC but she is too cowardly to actually back away from her, hence she wants to leave.
Gemma is probably the laziest person in the house as she seems to do no tidying up, she doesn't make her own bed, which Daniella bitched about along with Chris earlier in the week, and the only tasks she seems willing to partake in are those where a reward is given or immunity. Yet Daniella says nothing to GC because she is scared of her and what do a lot of people do around bullies? Make friends with them so THEY don't get picked on and they make sure that persons bitchiness and ire is focused on someone else instead.
She knows what she is doing and is hacked off Steph called her out, hence the crying and the victim routine. It might wash with some but I think most people will see through it.