I don’t have now my work computer, but what I have done is I just used GitLab copy address to clipboard (very similar to GitHub copy address) and pasted it after the command git clone "copy-paste here" mypackagename note the last argument of the git command, that is the folder where you want to clone, thus the name without the jl ending. Eventually it’s same if you just rename the folder to remove the jl ending.
Maybe this is related: Pkg.add() through proxy fails on Julia 0.5, Windows - #14 by pyrex41