I've heard horror stories about some GG. They can go both ways, some can be very unforgiving with tank mates, others are placid gentle giants.
When I first set up my 180 a few years ago I stocked it with a RTGG, 4 tinfoils, 3 balas, 3 cigar barbs, 6 clown loach and 6 dennison barbs, all fish were tiny, about 2" apiece.
There was no hassle in that tank from the get go, a great com tank. They started getting bigger so I got another 180 and transferred everything to my new 180, except the RTGG, kept him in the original 180 on his own.
Then I got a chalceus and put it in with the RTGG. He nearly killed it, he went mad on it. So I put the chalceus in my other 180, where it recovered and thrived. It was behaviour I had not seen before from my RTGG.
Fast forward even further when all my fish are now large. I got a 360 fibreglass tank to put them all in. I was very nervy at this time, I just didn't know if my RTGG was going to play nice or not. A little trick I did was put all the others in first, let them settle down for 24 hours or so, and then put my RTGG in. And you know what, he was as placid as anything. They've all been together for over a year now with no issues whatsoever.
I think my little trick was the difference between war and peace. Give a fish time to dominate their new tank, and then put strangers in, and it can get messy. The original fish looks on it's new "friends" as invaders, and treats them as such. But if you switch it round and put a potential aggressor in the tank LAST, then that can be key.
This practice is commonplace in the hobby, with lots of species, maybe it will work with you. Let your aro settle in first for a while and then add the GG.
I wish you all the luck in the world.