We have gotten off topic a bit, and yes I'm just as guilty in that respect. So lets get this back on topic. It is clear that the GrB 39 was available, and was fairly common. I feel it would be a very welcome, and balanced, addition to the game. The only thing comparable that the Russians had was the D-32 Rifle Grenade. Which was horribly outdated by this time, but at least had the advantage that it didn't require a special launching round be loaded to use it.
The Russians could be given the option of having anti-tank infantry equipped with a Mosin-Nagan and D-32 grenades instead of the PTRS. It could be useful in maps with no tanks. This would compensate for the GrB 39 being able to fire both AT and AP grenades.
But I do question how common they were...
"We received rifles and grenades which could be fastened onto the rifle barrels. Well we tried them but they proved totally worthless. When you pressed the trigger the grenade flew only about five meters and failed to explode. We then turned all that junk in." Vitjuk I. M., Red Army soldier
"Before commencing shooting you were to place the rifle and the grenade on its barrel on a firm surface on the bipod and aim at the enemy (we could have figured that part out by ourselves) and then shoot with a regular cartridge. It was then that I started regretting having sinned and now being forced to haul the launcher around as extra weight. I threw the contraption away because at no point in the war were we issued any grenades for it."
Bobkov A. A., Red Army soldier, 95th Infantry Division
These quotes would lead me to believe that they weren't very common, or well liked even when available.But for those who insist the Russians get something because the Germans do, then this is it. In spite of that, I'd still use them myself when playing the Russian side on maps with no tanks. For those worried about the so called "noob tube", firing rifle grenades requires the rifle to be braced against the ground, so you would need to deploy it in game terms. Rifle grenades are most certainly
not a run and gun weapon, unless you like dislocating your shoulder... ok, I'm exaggerating slightly. But they were almost always used by bracing the butt against the ground and firing at an angle like a mortar.