Note: I've tried to make sure I found the correct link for each project, but if unsure, please double check. Also, I have not bought from any of the links listed, so I cannot say anything about the quality of the different merch.
From your purchase we support free software, usually $3-5 from each product sold. But not everybody can accept money due to legal reasons, in this case, we give free shirts to developers.
I have a gentoo sweater. They have an official merch store, but it looked a bit rough. I, shamefully, got a made on demand sweater from Amazon with the gentoo logo on it. I did also donate the price of the sweater straight to gentoo though as well. I donate to them from time to time.
I got a Rocky Linux shirt directly from them. I didn’t think the shirt material was great as it was that kinda rough cotton but happy I was able to support. If anyone from Rocky Linux reads this, I’d buy another shirt if it was softer!