Gym culture in Barcelona

With the work visa done, I will soon be moving to Barcelona.
I have lived in a country with different culture before, way different. So this time I am trying to antecipate in some day to day cultural differences that may arise.

One of them is gym culture.

Here in Brazil, you can use any clothes as long it is within reason and not showing your private areas, and a locker room is for you the change and take a shower if you want. You should go clean to the gym so you don't make everything dirty, and it is polite to bring a towel to put between you and the machine.

I lived in Bosnia for a year, and gym there had its quirks.
Tanktops were forbidden, you must have a sneakers to be used only in the gym room (this is actually nice), and you absolutely must not get naked in the locker room.

What is the overall gym culture in Spain?

Also, is it easy to find a gym that opens at 5 am? or 6 am at least. I am an early bird.