From VG247: “Theres still time to earn Pinnacle weapons like the Mountaintop grenade launcher before the end of the Season of Opulence, so heres how to get them.
Pinnacle weapons were previously reserved for Crucible players, but now youre able to get your hands on two additional pinnacle weapons from Gambit and Vanguard strikes.”
Source: N4G PC Destiny 2: New pinnacle weapons announced for Season of the Forge