They also appear to have a fundraiser for the same thing here if you want to donate but don't need a shirt

https://actionnetwork.org/fundraising/support-rockstar-workers-fighting-unfair-dismissals

Or if you want to donate to the Union directly you can do so here

https://iwgb.org.uk/en/page/solidarity-squad/

I'm in no way associated with them, I spent all of 10 seconds searching for those links, there is some indication the two links might ultimately go to the same place, but either way I thought I'd drop them in the comments if there was anyone else like me that would want to throw them some money but didn't need another "thing" in return.