Neither Nightwatch or Trinary System are considered Gambit weapons for this particular Triumph.
This is the reason why the Drifter sells Bygones and Trust
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Aren't those both sunset though?
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Yes, but in addition to Bad Omens - these are the only two weapons that can still be obtained in the game. You can swap Bygones with Malfeasance if you have acquired this exotic. The only weapons that worked for this Triumph are the original weapons when Gambit was added at the launch of Forsaken. (The list of weapons can be seen in the Collection Badge - Playing for Keeps) For whatever reason, Bungie has never updated this triumph to reflect the weapons from Reckoning and the Year 4 ritual weapons.
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Much appreciated, I will continue to suffer with my newfound knowledge
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You're very welcome and best of luck to you.
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Thanks to your advice, I finally have the triumph done after 3 painful 4 stacked rounds. You're a godsend.
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Congratulations! I was glad that I was able to help to achieve it.