I didn't know at all (or just successfully forgotten) that git allows to include other config files.
While making a humble attempt at #700 I did RTFM to see

   The contents of the included file are inserted immediately, as if they
   had been found at the location of the include directive

So, with the sample git config files in #700 you would get

$> git config -f git/test/fixtures/git_config --includes --get sec.var1
fatal: bad config line 26 in file git/test/fixtures/git_config

$> sed -i -e 's,  gui ,gui,g' git/test/fixtures/git_config # for now   

$> git config -f git/test/fixtures/git_config --includes --get sec.var0
value0_included

$> git config -f git/test/fixtures/git_config --includes --get sec.var1
value1_main

whenever the test added in #700 (https://github.com/gitpython-developers/GitPython/pull/700/files#diff-1d9cdcd948df3c80edc698aac95bfa27R100) reveals that GitPython does not "load right away". Since I felt that it is a bit of a big RF to do, didn't attempt it yet