It appears in Northern Kyrat as a fixed and vehicle mounted weapon, as well as becoming available as a hand held option later in game.
It is a good weapon if you value firepower over subtlety, but otherwise is very much a 2nd choice weapon for most scenarios. It is often ignored for M79, as it is used as a sidearm, and two grenade launchers is a loadout that likely isn't something you would need. However, the GL-A87 is somewhat better for use against tougher vehicles and helicopters because of its ability to fire automatically. It is especially good for taking out clusters of vehicles, and can wreak havoc on Pagan's Wrath convoys.
References EditFirst Appearance: At 3:38 on the link, there is a brief segment showing the GL-A87 shooting at a helicopter. https://www.youtube.com/watch?v=7g-MgJXACkU