CoinGate

Log in to your CoinGate dashboard and open your API settings.
Copy your API token.
Add it into the admin panel under Settings > Payments > Processors > CoinGate > API token.
Set Settings > Payments > Processors > CoinGate > Mode to match the token you are using. Use Live with live tokens and Sandbox with sandbox tokens.
You can switch between live and sandbox API keys whenever you want to test, but sandbox tokens work only with sandbox mode.
In your CoinGate callback settings, make sure the callback format is set to JSON.