First of all, you have to get the Tarball gajim-x.y.z.tar.gz.
Then you extract the tarball archive (without the debian folder) and add the debian folder:
tar xvzf gajim-x.y.z.tar.gz
mv gajim-x.y.z.tar.gz gajim_x.y.z.orig.tar.gz
cp -r path/to/debian gajim-0.7/
Then, before building the package, you run dch in the 'debian' folder to update the changelog:
dch -v x.y.z
dch -i to build a new package of the same release, if it's not a native Debian package, or to increment the version number if it is a native Debian package.
dch -v version to specify the versin number (dch -v 0.3 for example).
dch -v version
dch -v 0.3
All the necessary information to build the package is included in the subversion repository.
So, to build it you only need to install the necessary packages (python-dev and libgtk2.0-dev), and then run:
in the gajim folder.
After that, you get the binary and the source packages in the parent folder.