It was a while ago, and I remember I did a lot of things. For some ideas, see
http://stackoverflow.com/questions/8329485/git-clone-fatal-unable-to-find-remote-helper-for-httpsI remember that installing curl-devel finally did the trick.
Then I removed and reinstalled the latest version of git from sources. I just now tried the "git clone g...@github.com:nsf/gocode" version of the command and it all worked. And 'go get' has worked well for months.
I'm now running git version 1.8.4.3 under Ubuntu 13.10, 64-bit, with the new go 1.2. This combination works very well for me. I installed the latest git by compiling from sources, which I recommend.