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D-Gate is a D-App that is deployed on Binance Smart Chain to serve websites as a payment gateway. Make sure you've installed MetaMask wallet, it's needed to interact with the blockchain. You can create a test app with fake BNB (on Testnet) and test your app here: https://testnet.binance.org/faucet-smart-wallet-enabled-to-test-smart. After a successful payment, the gateway shows you a link to get back to your test app and you can see the invoice is paid successfully.