I don't really notice it much, honestly. And you're right, you don't need to dress up for the gym. However, when girls do dress and look cute--especially if they have a nice body to show off, I definitely notice. Nothing like yoga pants and a form fitting shirt over a great butt and big firm boobs to grab the attention! Hahaha!
I don't go to gyms, especially unisex gyms. I figure if I did women are going to go and wear things that would distract me and I don't want to hurt myself because I'm looking at a woman's bum or whatever. Wear what you want butttt just know that the gym isn't really for meeting people like that
I don't think of the gym as a place to meet girls, but a pretty girl will look good in anything. Just wear something appropriate for a workout; if you dress like you're going to a party it is fake and obvious.