It's not the most substantiated claim, but it's decently believable. She's been scared of guys all her life, now she has a reason to like them, and she delightfully surprised. As you say, no proof, but I shall see what upcoming comics say about the hypothesis.
Yes, Yuki not offering returns is hardly fair, but consider that she's been traumatized to the point of hallucinations. I can fully understand why she's not offering, and since Gary isn't asking...
Which is why Gary needs to toughen the heck up. For both of their sakes. Exposure and careful acclimatisation might be the key to Yuki overcoming her phobia, and Gary's benefits are obvious. Otherwise, he'll need to overlook his options (by this point they are considerable) and find a new damn girlfriend.
I'm not saying Gary isn't a newbie at this. I'm not saying he shouldn't be treated with care and expected to not be the most smooth operator. But he has to work on getting better. He can't expect others to do his work for him. And he (and we) can't forget that he isn't the only one in an unenviable position.