when to use API Gateways versus direct microservice calls

So, I've been pondering microservice architecture. When services need to talk to each other inside the system, most people just use gRPC or REST calls, right? But when should you actually bring in an API Gateway? Is it just for outside folks, or are there good reasons to use one internally, too? I'm really interested in how you guys handle your service-to-service chat and what makes you decide to put a gateway in the mix. What are the pros and cons?