Make microservice more secure (by hiding it)?

I build an application with gateway and microservices. The microservices need a secret key to answer. That secret key is setup in gateway. So every request must be done to the gateway and the gateway redirects the request with a user id to the right microservice. Is it possible to make another security layer to it? Im not a person who knows a lot about hosting, but is there a way to build a network with multiple servers and then only allow requests at microservice from the network, where the gateway is in? Can I build this with a tool like laravel forge? ... If this makes all no sense what I said... is there another way to have more security, or is a secret key secure enough to start?