Hi Jorden,
I wrote a howto to install 2.7x from git about a year ago.
viewtopic.php?f=24&t=1261It contains a lot of info on problems that might arise.
Also on compiling tar.bz2 files.
As for your problem gimp configure not finding gegl:
If you have "mc" installed go to /usr/local/lib and open it.
It should contain /pkgconfig.
Open it and check for gegl.
If you don't have mc then try this:
ls -a /usr/local/lib/pkgconfig | less
Less will give you the contents of pkgconfig page by page.
If gegl is not there something went wrong during gegl's install.
Do you want to keep Gimp-2.6?
My experience is that having both 2.6 and 2.7 in the same install can give problems.
That's why I have it on a separate install.
Good luck,
Gerard.