Yes, Gracie is totally fine with guys wearing girls clothes. Gracie doesn't even notice gender -- she only notices the themes of the clothing put on. That's why you can get away with wearing stuff that is really atrocious, and not fashionable in the slightest -- because she's is only programmed to see the theme of the pieces, and nothing else.
The reason why modern is difficult, though, isn't just because most of the stuff is sold at GracieGrace, though, that certainly doesn't help matters much. The reason is also because there's so little clothes that actually have the modern theme! There are lots and lots of shirts, and often, when I see people asking for modern clothes, they already have the shirt down. However, there's only one bottom -- the dark polka skirt. There are two dresses, the cool dress which is sold by Gracie, and the blue retro dress, which I'm pretty sure is currently out of season at the Able Sisters. Both of the hats are sold by Gracie. There are only two accessories, one of which is sold by Gracie. And there are only two pairs of shoes, and again, one of them is only sold by Gracie. There are no modern socks. Honestly, if there were more pieces that were considered modern, it wouldn't be as difficult to pull off. But with only two hats, two accessories, and two pairs of shoes to choose from... Yeah, that's pretty difficult.
The good thing, though, is that you don't have to stick to just modern. I think as long as you have three pieces, you can get away with wearing things from other themes, provided they don't clash. (Actually, correct me if I'm wrong, but I don't think anything clashes with modern...) When I got modern in my cycling town, I paired it with iconic, and got a really nice review.
So, if you are stuck for things to wear, you could try mixing it up a bit.
I have a shirt, pants and shoes, but I can't find socks, a hat and an accessory...
