EDIT: Since the well known "mission patch" this method is now being debated as defunct. This has not been proven nor un-proven, but is in constant discussion. I would suggest reading through the most recent replies before you decide to follow this method which worked for me before the missions changed so dramatically, along with (apperently) the loot tables.
I have had a bit of luck with finding GA. So far I have found on my own, GA1 and GA4. I did not "stumble" on it, or even camp for weeks. I intentionally got missions aimed at finding it.
Here's my methods, that seem to have worked twice, and I am still trying to find GA3, btw.
I join a team, and do a team mission (full team is best). But, before heading to the mission, I visit a mission terminal and grab my own missions to solo for later. You can do this with team's permission if you like, they usually don't care unless they want to line up more than one mission. I usually go to a diff zone and roll so people in my team don't get key confusion.
Here's the level rule. /tell helpbot <yourlevel> to see what you can roll. Discs are QL 60 for GA1, 93 for GA2, 116 for GA3, 140 for GA4. Take the mission one "set" lower than the target QL. This means, a QL 109 mission will have roughly a QL 116 boss, hence higher chance of a 116 disc being on the body. Edit: The reason you do this in a FULL team is that a "Full Team" mission boss drops more loot than if you are just teamed with one other person. =)
(On a strange sidenote, I found my GA4 off a QL 117 boss, so it's not an exact science.)
The mission settings I used both times was Full Bad, Full Order, Full Money, and the Phys slider towards Phys at 25% (for the whole bar) ---|---'------
So then all you do is blitz right to the boss ignoring the mobs and hope your GA is on the remains. For GA1 I killed maybe 15-20 bosses. And for GA4, I went through about 20 or more. I think camping and dynas are overratted and missions are much more interesting.
But anyways, hope all that helps, and if you're a camper, might relieve some stress from the routine.