If you are not using one of Stripe's SDKs (like Express Checkout Element) please make sure to set the correct params within the tokenizationSpecification object. For Stripe this would look like this:

  "gateway": "stripe"
  "stripe:version": "2018-10-31"
  "stripe:publishableKey": "YOUR_PUBLIC_STRIPE_KEY"

https://developers.google.com/pay/api/web/guides/tutorial#tokenization