Theta gang just means someone who sells options and expects that option to expire worthless, thereby profiting from the slow decay of option value due to theta. It's a legitimate strategy in the options world for professionals, not just for WSB autists. Here's a random example of a finance professional running this strategy successfully in his personal account: https://earlyretirementnow.com/2019/03/27/passive-income-thr...
Disclaimer: I don't know this person, nor do I recommend copying his strategy if you don't know the options greeks; actually the amount of money you would need to execute his strategy would probably make most options beginner uncomfortable anyways.