You can *always* compile from source, but unless you are comfortable with the ins-and-outs of it, you will probably end up tearing your hair out in frustration.
The thing about using Mandrake RPMs in Mandrake is that you can be reasonably sure it works, and it is a "point-and-click" type operation.
If you *really* have to have the latest Gimp before there is a Mandrake RPM available, here is how you go about it (in general terms):
1) download the source tarballs (<filename>.tar.gz)
2) extract the tarballs (tar xvzf <filename>.tar.gz)
3) cd to the newly-created directory
4) enter the command "./configure"
4a) resolve any problems from the configure command. This might include installing additional software and dependencies. Repeat steps 4 and 4a until no errors are reported.
5) enter the command "make"
5a) resolve any problems from the make command. This might include installing additional packages and dependencies. Repeat steps 5 and 5a until you get a clean build.
6) enter the command "su -c 'make install'" (There *shouldn't* be any problems if you get this far, but if there are, you need to resolve them before you can use the application.)
7) Find where "make install" put the program, and set up any menus to refer to it for ease of accessing.
(Just a bit more hassle than typing "urpmi gimp".