Are you talking about Google Play (where you purchase apps and games for Android) or the Google Store (where you purchase Google hardware)?
Stadia payment methods are configured through Google Pay. When attempting to purchase a game through Stadia, I do appear to have an option to use my Google Play credit in addition to the cards I've added to Google Pay, but I don't see a way to apply Google Store credit towards a game purchase.
@Exar So, for science (uh let's go with that.. lol) I tried to purchase a game from the app. It's asking me for payment methods, and at the bottom, there's an option for 'Redeem Code'. When I click it, it's asking me to 'Redeem a gift card or promo code'. So I'm not sure if Google Store is considered a promo code, but looks like it will work?
If you want to be completely sure, you can try contacting Stadia support:
Go to the link below:
https://support.google.com/stadia#topic=9634925
On top right, click on 'Contact Us' > then request chat/callback to speak to a representative. Try it from a computer browser if the option is not shown on mobile