Gatling authentication with github

I an deployed gatling on EC2 instance and I run the test using the frontend. However while it is trying to download the test scripts from my Gihub repo it is showing authentication error. The issue is that Github does not support username and password auth anymore. I know I have to configure it to use ssh and I need to add Github token but the provided box to add ssh-token is greyed out. Please how can I configure this to allow me use environment variables?