Is it possible to use ALB/NLB infront of an API Gateway.

Hi Arjun, Why would you want this kind of architecture? What are you trying to achieve?

ALB is possible behind API Gateway, not the other way around.

You can put both an NLB or an ALB in from of a REST API, as long as the API is a private API which can be accessed only from the VPC where you place the load balancer. You configure the load balancer to use IP address for the target group and you specify the IP addresses of the API Gateway VPC Endpoints. The following extrenal blog shows how to do it for creating custom domain names for private APIs.