Hello, I want to restrict access to an api gateway for only a specific other lambda function. is there a way to do that?

Do you mean you only want that lambda to be able to send requests to it?

yes exactly @thdxr

I'm not super familiar with this but I believe you can use an IAM Authorizer on the endpoints and specify that the lambda function is allowed. I think this area in docs is a starting point: https://docs.serverless-stack.com/constructs/Api#adding-auth