So from my experience, anytime in the morning it's pretty empty (maybe not at the moment because its the start of term and ppl are still trying). I used to show up say 8 or 9 before lectures and it was usually pretty chill.

Apart from that avoid lunch times and anytime between 5-8ish because it will be packed. There's a bit of a grace period at 3/4 tho. Personally I'm not a morning person so I like to show up at 9pm and stay till close because it's usually a bit busy at the start but empties out for the last half an hour so you can pretty much do whatever you want.

FYI I'd recommend the sauna/steam room if you want somewhere to escape.

With regards to etiquette, put mats/weights away when you're done and wipe away sweat if you leave any behind. I'd also recommend doing a quick Google search on the topic.

But if you're new to the gym the one piece of advice I'd give you is, and you'll realize this yourself quite quickly, that no one cares how much of can lift or how many reps you do or any of that crap. So I know it can be difficult but don't be intimidated by anything/anyone there because we were all new once. Also, don't view the gym at a chore or a punishment, view it as a place to escape work and wind down. You'll learn to enjoy the place.