Use their gun skill, or if you want, gun skill - 1. Troops issued rifles with attached grenade launchers still have to qualify with the weapon (in the US Army at least). If you want to assume good skills, then use their gun skill. But if you perhaps you felt they would not get that much training, then the N-1 would work as well. For those troops who weren't infantry, or perhaps they had the ground forces designation, but road a desk to Admin-4, then go with no positive DM at all. They knew the launchers were around, they fired them once a year, but that was about it.
Dedicated grenade launchers should, in my opinion, also go with the gun-combat skill. Today they are essentially the 'rifle' for the grenadier, and it becomes his primary weapon. I don't like lumping it in with launchers because I equate them with something more shoulder-fired than a rifle-type weapon. But that's just me. Skills like Heavy Weapons should be applied to something that is not normally used as a rifle. Using today's tech, an assault rifle, a grenade launcher, a squad-automatic weapon (SAW) would use gun skills. A shoulder-fired rocket launcher, a 40mm auto-grenade launcher, or a .50cal machine gun would be heavy weapons skills. It's arguable where exactly something like an M60 would fall in... it kind of straddles both definitions.