GOPATH has nothing to do with GitHub. It is one of the places that has special case handling, but is absolutely not the purpose of GOPATH. Case in point: the VM I have to build some legacy stuff using GOPATH does not have a _single_ dependency from GitHub. Not one. Nor does it contain any open source code, besides the standard library.
The logic of GOPATH and "go get" just so happen to revolve around building packages, fetched from source code repository. This logic used to be a stop-gap, written during early stages of Golang development, and makes it awkward (at best) to distribute proprietary Go libraries. I assume, that Google is introducing module system (and aggressively deprecating GOPATH) to work around this issue (among others).