I'm afraid it's not enough to have the dependencies in a folder. The debian packaging system Ubuntu uses requires the dependencies to be installed in the system. Each installed package is added to a database.
A possible advantage of this is that the dependencies' programs and other files will be copied to the directories that are part of the system's various search paths so other programs you install later will be able to use them too.
If you have to install the debs manually you can examine them first with the archive manager (just right click the deb's in your GUI and you should see it). Look at the DEBIAN/control file to see what dependencies it has. Then you can try to install just the dependencies you need. If you get an error message about dependencies when you are installing a dependency you may need to recurse.
Gimp is so complex that you may have to take your system somewhere that has an Internet connection, unfortunately. Gimp spawned the toolkit library gtk that is used all across gnome GUI programs (many, many Ubuntu GUI programs). Fortunately, all the basic ones should already be part of your system if you are working on an Ubuntu desktop system.