According to the gRPC.io website I know gRPC supports http 1.x and 2.0 but what about other "transports" like zeromq or nanomsg?
gRPC does not support zeromq or nanomsg. The supported transports are HTTP2, QUIC and in-process. You can find more details in https://grpc.github.io/grpc/core/md_doc_core_transport_explainer.html