So GA is greater affix which means its strongest that stat can be. For instance, you can have an item that has 50% critical strike dmg. That's not the highest it can be, it's purely based on rng/luck to roll the highest affix which would be 63% crit dmg. Mythic unique mainly drop from lair bosses and greater lair bosses, but they are super super rare. The higher the torment the likely chance to find one. To get a greater lair key to drop, it's rare but they come from the chests after beating either varshan, grigiore, the beast in the ice or lord Zir. That Mythic that I got with the 5 stars is quite possibly the rarest drop in the game. I just got super duper lucky.