You can wear whatever you want, there are no rules.
I would say, though, that I wouldn't call it a "gypsy" costume because that word is pretty loaded - like if you said you had an "indian" costume - and while it's totally awesome and cool to study historical people and understand their cultures and appreciate them, it can really hurt those people and those that care about them to see a negative name for their group applied to a fantasy costume.
Even just saying "I'm a fantasy (*finger quotes*) gypsy (*finger quotes*)" will let people know you understand the difference. There's nothing wrong with your costume
- so don't take it like that.
On another note ! It will most likely be super hot and sunny out - if the one you're going to is anything like the one I go to you will likely be really happy if you can incorporate a brimmed hat of any kind. And make sure to wear sunscreen. Also, again, if it's anything like the one I go to it will be really dusty so you will get pretty filthy. When I worked at one for a season the costume I wore... well, I'm pretty sure it will never be actually "clean" again. Also, if the ground is dust/rocks/dirt/woodchips you might be much happier with closed shoes. Some pretty gross stuff gets on the ground and just for the squick factor I wouldn't want my feet exposed to it.