is all GA standing room only? what is the difference between GA and GA5 seat 8

Yes

I've only gone for soccer so GA is only standing

When I went the back half of GA had fold out seats.

The show i went to had 4 different GA sections, each is barricaded from the next. The only one with seats was the rear section. Not sure if it will be the same for your show or not. One warning: everything about this venue is RIDICULOUSLY overpriced. Parking is 40 or 80 dollars! Beers are at least $17 so be forewarned this place is corporate greed at its worst!

For LAFC games, all tickets in the North End (first level) are GA. They are a standing section (seats are locked out of the way). The GA vs. GA3 vs GA5 designations are meaningless for the attendee. Guessing they are for the ticketing services only.