Take a look at what Alex posted:
That is as close as you will find to a Galileo repository. It does include a gcc_runtime_dbg.
And clearly you can not build gcc on the Galileo from source - since that would require a working gcc.
If I were you - I would give up on the provided SD image, and download one with dev tools from one of the posters (Alex maybe) on this site. Going the package route will take you days, just figuring out all the pieces you need.
Very nice summary :-) I have almost nothing to add, except that generally building devtools as packages is possible, I just haven't yet come to that focusing on the devtools image itself. But packages are a byproduct of that, I'd just need to build the same for 1.0.0 and then upload the packages to the repo.
For a literal answer to your question - see this thread, there's a package repo I support (inofficially): How to enable the Yocto-provided package management capability
As far as devtools are concerned - in the absence of those in a form of packages, check out this image, it has the tools built-in and is based on the standard 0.7.5 one, so you lose no functionality using it: Collaborating to develop a "devtools" version of Linux which is Arduino IDE compatable