GooglePayGateway
Google Pay — a wallet method riding a payment gateway underneath (here, a "example"/TEST tokenization gateway; production would set gateway = "stripe" + the real Stripe account's gatewayMerchantId, since Google Pay hands back an encrypted token for a processor to charge, it doesn't settle payments itself). ENVIRONMENT_TEST needs no live merchant account, so this ships SANDBOX_READY — genuinely runnable by anyone with a Google account and a test card on file.
docs: https://developers.google.com/pay/api/android/guides/tutorial
Request-JSON shape adapted from khalid64927/google-apple-pay (Apache-2.0) — see GooglePayRequestBuilder doc for why that repo can't be depended on directly. The Activity-callback → coroutine bridge below is this module's own pattern (mirrors com.siddharth.kmp.paymentsapi.AndroidPaymentHost usage in the Razorpay/Stripe providers), not ported from that repo's Fragment-based resolver.