API gateways are ingress middleware that handle various tasks such as service discovery, load balancing, rate limiting, TLS termination, etc.
AWS went ahead and named a product “API gateway,” which caused me to believe that it was an AWS-specific product. However, there are open-source API gateways, such as Kong. GCP also has an API gateway called Apigee.
Briefly discussed in (Xu) System Design Inteview, vol. 1, ch. 4 (“design a rate limiter”).