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I Did NOT wrote a single word of this Tutorial : is just copied and paste from
http://www.gimpusers.com/news/2009-04-23/gimp-master-for-ubuntu-9-04.html
But i think may be useful so i post here at it is
And i hope somebody may offer something similar but for Windows...i hope from 3 years but till now nobody seems able to fill the gap

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UPDATE 3 may 2009
There is a updated guide to compile Gimp-2.6 on Windows is here

http://wiki.gimp.org/gimp/HowToCompileG ... oftWindows

i did not try yet but at a first glance this time the how to seems detailed and fully updated



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Here you can see how to fetch the current GIMP source code from the Git server and compile it for Ubuntu 9.04:

1. The new files for GIMP 2.7 shall go to /opt/gimp-2.7 so they don't disturb the GIMP version installed by the system (GIMP 2.7 in case of Ubuntu 9.04). Both versions shall be usable at the same time. So you need to set these environment variables on the command line (and later use the -prefix parameter):

export PATH=$PATH:/opt/gimp-2.7/bin
export PKG_CONFIG_PATH=/opt/gimp-2.7/lib/pkgconfig



Now switch to a temporary directory and fetch BABL, GEGL and GIMP from the Git server:

mkdir ~/tmp # if not existing yet
cd ~/tmp
git clone git://git.gnome.org/babl
git clone git://git.gnome.org/gegl
git clone git://git.gnome.org/gimp

Some libraries and its development packages (including headers, API definitions etc.) are needed in order to compile BABL, GEGL and GIMP. You can install the most important packages using this command:

sudo apt-get build-dep gimp

This command installs all libraries and packages that are needed to compile the GIMP version shipped with Ubuntu (= GIMP 2.6). Because the dependencies of GIMP 2.6 and 2.7 are mostly identical, the most important libraries are installed now. However, you have to install these additional packages:

sudo aptitude install libtool ruby

Then you can compile and install BABL, GEGL and GIMP:

cd babl
./autogen.sh --prefix=/opt/gimp-2.7
nice make -j3 # 3 instances, suggestion: number of CPUs+1
sudo make install

cd ../gegl
./autogen.sh --prefix=/opt/gimp-2.7
nice make -j3
sudo make install

cd ../gimp
./autogen.sh --prefix=/opt/gimp-2.7
nice make -j3
sudo make install

Now GIMP 2.7 should be installed. You can launch it with /opt/gimp-2.7/bin/gimp - have fun!
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EDIT
2 useful suggestions (QUOTE FROM http://www.flickr.com/groups/gimpusers/ ... 207532628/ )