My gas cap strap broke a long time ago, and I finally got around to replacing it with one that I got on AliExpress for just $1.50.
https://www.aliexpress.us/item/3256806298929545.html
That's the one I ordered, but if the link doesn't work you can search for "16117222391". I got just the strap and ring, and the original ring was a bit hard to get off but I just broke it off with needle nose pliers being careful to not damage the gas cap itself. The new ring was also quite hard to snap on, and once in place, mine was too tight to freely rotate around the gas cap. So if you want to avoid these issues, just get the whole cap and strap for around $5.