From GameWatcher: “The Notorious Armor in Destiny 2 is a notoriously hard-to-get piece of gear. It is necessary if you’re serious about picking your role in Gambit, so we suggest you take a passing interest in getting it.
The whole shebang kicks off as soon as you upgrade your Mote Synthesizer, which in turn allows you to start unlocking the highest tier of Gambit armor. Not only does the Notorious set look great, but it’s also a big boon in Gambit modes.”
Source: N4G PC Destiny 2 Notorious Armor – How to get Gambit Prime Gear?