Gosh is a reimplementation of mosh, but not protocol compatible with it. It embodies the same ideas - secure UDP connection, IP roaming within the session - but does not implement every thing mosh did. It will do things mosh doesn't though, like ssh agent and port forwarding.
To build Gosh, you need a go compiler that is newer than the one specified in go.mod (1.22 as of this writing) and a protoc with the go protoc-gen-go that is new enough to handle edition 2023 protobuf definitions.
You should be able to pull the protoc-gen-go, if needed, from google.golang.org via:
go install google.golang.org/protobuf/cmd/protoc-gen-go@latest
The protoc compiler, if not new enough your machine already, is available in binary form from https://github.com/protocolbuffers/protobuf/releases
With the go, protoc and protoc-gen-go binaries in your PATH, a simple
make should suffice to build the three primary binaries (gosh,
gosh-server, gosh-client).