You don't have to or are being forced to at all. It is not pvp it a 50/50 that requires teamwork but people in this game still don't know how to play control so i don't know why bungie thought they could play gambit much less gambit prime with roles. Pve mains or purists are very stubborn elitests who get bent outta shape in pvp because its not the instant death button or i can out lvl simulator. Pve purists are hardly teamplayers with small exceptions and i mean really small exceptions. Gambit was both a success and a failure. Success in adding a unique take to gambit and speed up time so the game isn't drawn out. Failure to accommodate a type of player that knows nothing about opposition or battle tactics. They should have made a really hard pve dungeon that gets progressively harder the deeper you go with high risk high reward.

A) Gambit prime is 50% PvP at most B) Apart from unlocking the tiers, Gambit is in no way required to do Reckoning. I mean, if you want good rewards from it, you will need to get synths which are best obtained from Gambit Prime, however there are a couple of bounties that give synths without any Gambit C) There is a very clear in game reason why Gambit Prime is tied to Reckoning. To put it simply, it’s Gambit mans ship, drops gear related to Gambit, so of course, Gambit is involved