gRPC encoded data is actually not tightly coupled to HTTP/2 frames. It's described as a stream of gRPC chunk encoded data on top of a HTTP stream. But the gRPC frames do not necessarily have to align with HTTP/2 data frames boundaries.
The specification makes gRPC sound rather tightly bound to HTTP/2 level details, but I don't think it really is. You should be able to speak gRPC just fine over any kind of HTTP. One main pain point for browser support however had been APIs to exchange trailers, which are necessary for gRPC.
See https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2....
The specification makes gRPC sound rather tightly bound to HTTP/2 level details, but I don't think it really is. You should be able to speak gRPC just fine over any kind of HTTP. One main pain point for browser support however had been APIs to exchange trailers, which are necessary for gRPC.