it's not searching for "gimp-2.0" it's searching for the pkg-config file ( the numbers do not have to match, for example gtk+-2.6.x will have a pkg-config file called gtk+-2.0 not gtk+-2.6).
cat the package you installed ( cat /var/log/packages/gimp-xxxxxxxxxxx) and look for a $PREFIX/lib/pkgconfig entry, you need to know if the package you installed came with the .pc file configure is looking for "gimp-2.0.pc"\
if it didn't come with the pkg-config file, you will either have to find the rest of the package, get a decent package or build it from source.
edit: just noticed you installed via checkinstall, does that mean you compiled it yourself? where did you install it? did you pass options to configure?
what you probably need to do is ( if you did ./configure, make, checkinstall ):
Code:
export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:$PKG_CONFIG_PATH
and re-run configure