It depends on what you want to do.
If you get "girl clothing", yes it will most likely be obvious that it's girl clothing. You might not notice it fully if you're not used to looking for differences - but there will almost certainly be a huge difference.
There are some types of clothing with a more "unisex" cut - but you have to take it case by case.
For instance, if you want to do a crossplay as a boy and look like a boy and that boy wears a suit jacket - a women's suit jacket is going to look totally and 100% "female" and you won't "look like a boy". It will be obvious even to people who don't know what to look for in the jacket.
People are exposed to gendered clothing every day of their lives. Even if they couldn't write you a list of the differences or "put their finger on it" they can still tell.
If you just want to do a costume of a certain character and you don't mind doing a "girl version" of them or if you don't care if you "pass" or look like a boy then it wouldn't be a big deal to wear "girl clothes".
It depends on what you really want to do.
Disclaimer : This post written by a Grumpy Old Man
Actually in the pipes:
- Remaking Ser Loras doublet.
- Luke Skywalker, A New Hope
- Wind Waker starter costume in swim material
Eventually: Paul from Dune, Faramir, Harry Sullivan