The compensator is described as:

Reduces recoil. It is incompatible with suppressors.

It’s available in northern Monte Puncu at the very top of the map. The accessory case is in a small compound at the end of the rail line.
There’s no resistance here so once you reach the area, you can just pick up the accessory.

The compensator muzzle reduces recoil which is great and this is so easy to collect if you’re in the area anyway you may want to try it Although you may prefer to grab the Compensator V2 for LMGs available in Montuyoc which reduces recoil further.

The two downsides are; it slightly reduces noise reduction, and when used, you can no longer equip the suppressor on your gun. But this is where compensators really make sense, is for LMGs. Unless you run with the Stoner LMG A1 (found in Pucara) which is the only LMG you can equip with a suppressor, you should definitely be using a compensator on your LMG in my opinion.