This might be simple, but it took me hours to figure out the full solution, so I put it here for your reference.

Suppose your internal gitlab host has IP or URL $GitHost, then set environment variables below:

GO111MODULE="on"
GONOPROXY="$GitHost"
GONOSUMDB="$GitHost"

And configure git as:

git config --global url."[email protected]$GitHost:".insteadOf "https://$GitHost/"

When parsing a package import path, say your.internal.git.host/package, Go would first try https, as https://your.internal.git.host/package. This would be caught by the git URL conversion rule, becoming [email protected]/package, which is serviceable by gitlab via SSH (You should have your SSH keys configured). The “NOPROXY” and “NOSUMDB” tells Go don’t try to access through proxies(defined by GOPROXY) and don’t verify checksum from a global database(defined by GOSUMDB).

Somehow the still-ugly part is package names should be ended with .git, as defined by gitlab, but I think this is acceptable.