Her Q is enough to stick to him. Even then, if he burns his E wrong once he's a dead.
Either person can get screwed there. If Fiora Qs in, she's committed because she has zero escape from that point and if Gnar leaps off of her head or a minion, he's safely out, but she's vulnerable to a gank for a while if she had to push in to get to Gnar.
I love Fiora and I do think she can be a good match up against Gnar, but she's riskier than other melee champs. She has no stun or escape to rely on to get her out of trouble. It's up to the Gnar to make the mistake to win a fight 1v1, I feel. Someone like Renekton, Riven, or Jax can do a better job of handling the little runt.