I'd stay away from core gaming peripherals, that means stuff like mouse/keyboard/headset.
Those guys probably already sold a kidney for those things, as others members pointed out already.
Perhaps look into the following: microphones, gaming eye wear, headphone and cable stands that have USB plugs, video capture cards for those who might want to start a youtube channel or submit their 'top plays', sound amplifiers (they work with headphones to increase max volume), PC case peripherals, that means RGB controllers/cables/fans/cooling systems/other non-essential case hardware, gaming controllers, which isn't limited to those console-like controllers, I'm talking steering wheels, pedals, transmission boxes, flight simulation controllers (they are compatible with any games that include helis/planes but also any vehicles, I've used one in the battlefield series), attachable controllers for mobile phones (it's a thing), perhaps HD webcams because every kid wants to be a streamer now
Also those fucking VR things, you can look into that but I'm not sure how much an average gamer is into that, the technology is still in it's infancy and not to many popular titles have VR support if I'm correct.
From my experience going affiliate with some asian brands of affordable mechanical keyboards could work too, since gamers on a budget have been eating those like crazy lately, and the only 'legit' other choice they have are expensive razer mech. keyboards that cost like 2 or 3 times as much, but be aware that that market is (my subjective opinion) decreasing since gamers have reached a high point of awareness of these affordable Asian brands, you wont surprise too many guys by trying to pitch them some of those, but I can of course be wrong.
best comment righ?
EDIT: you can also look into custom printed mousepads, featuring art related to popular gamer culture, maybe dropshipping or perhaps joining forces with a manufacturer, I know those are popular in card game and anime circles, could go well when pitched to gamers I guess, fuck who knows