Does WAF support ALB with the new gRPC feature?

The gPRC setting is set on the target group on ALB, and it utilizes HTTP/2 for transport, so simply put - Yes.
You can certainly attach a WAF policy to your ALB, but as for the nature of the actual payload in gRPC I believe there is a risk of triggering false positives (when WAF will match a signature to some pattern in the request as it comes in).