I'm not sure how Garmin works, but for instance with Google Wallet-compatible watches, you need a phone where wallet can run. I've had this setup for a year where I loaded the cards from another phone and used a watch to pay.

However Wallet didn't like this setup. Tokens expired at varying delays, sometimes a day, sometimes a week or payment failed without reasons.

Nowadays, I just use my bank's app which work fine on GOS.