I think gunner was used as a generic catch-all for all shooting enemies, but unfortunately there are also are specific elites that have gunner in the name. The game uses shooter to refer to almost any enemy who shoots a gun (exceptions being Scab Ramboes and Ogryn Reapers).
Reapers would actually cover their head if you suppress/hit them enough with ranged weapons.
So I think most of the issue is people just not knowing which guns suppresse well.