I bought tickets for the balcony and the section is GABAL and the seat is GA, so is it first come first serve or is it a specific seat?

A ticket will list specific seat numbers on your general admission balcony tickets, but the venue does in fact adhere to the first come first served ok

That's correct depends on concert and if your tickets have a specific section and number if its not designated then no but you can always ask once arrive.

It depends on the concert I went to a concert and they had assigned seats

You can sit anywhere you like with those tickets. There's no assigned seats with GABAL. I went to a show about a month ago with general balc and it was easy finding seats and you're free to move around. Enjoy!