Click on the Streak dropdown in the top right navigation bar of Gmail and find the "Integrations" button.
Click on the button for Integrations.
This will open a window where you can view existing integrations and create new API keys. Treat your API key like a password, as it has full access to your account. If you feel it has been compromised, you can always delete it and create a new one from the dialog.
Keep your API key(s) secret.