Here’s an option for bismuth, these are in stock
https://bossshotshells.com/products/...mag-shotshells. These work very well but are expensive.
I would choose 7 or 6 (probably 6 for Gambels). I’ve had mixed success with steel shot in 20ga for quail. In my experience, an ounce steel works well but that’s a 12 ga load. Now days I use an RBL “Fox” in 12ga for this purpose.
PS, I mean no disrespect to anyone or to the great 20ga or to steel shot!
Best,
Brett