If you try to customize grenade launcher at gunsmith in game, you will see this massage only.
but Ghost Recon Network app(iOS, Android) can customize the grenade launcher's paint.
GRN App can change secondary weapon to handgun only.
so you should select grenade launcher as secondary weapon in the game.
it will be applied in game after sync.
Mk40GL's paints are hard to distinguish.
and it cost a credit for underbarrel item at first time.
I wonder why UBIsoft locked this function in game.