When should we think about using API Gateway for how our internal services talk to each other?

So, we're making our microservices bigger, and I'm really starting to wonder about how all these services chat, especially with more and more dependencies popping up. Obviously, an API Gateway is super important for outside APIs, handling stuff like who's allowed in, how many requests they can make, and sending requests to the right place. But for our internal service to service talks, when does an API Gateway, or maybe even a service mesh for some of these issues, actually help more than just calling each other directly or using a basic reverse proxy? I'm wondering if anyone has run into slow performance or things getting too complicated with direct calls when systems get bigger. And at what point does an internal gateway actually give you more than just a headache?