Are you able to visit at night/already dark to see the city lit up? Seen that it closes at 6/7pm. Was wondering about after those times.

Sure can, infact the view is incredible @ Sunset!

I went daytime and there was a sign saying it closes at 7pm. Since sunset is so early now your best bet is to go just before closing time to get a good night view

Can you go after its closed? Like is there a gate and lock that prevents you from going up?

Yes

Yes it closes at 7pm

It closes at 7.