When you get to the login screen, will the system allow you to change the default session to Gnome?
If so, simply boot your pc and then when it gets to the login screen, change the default session to Gnome before logging in.
Once you get to the Gnome desktop, as long everything still works properly without crashing or anything, then you should be pretty much sorted.
If you are unable to change the default session to Gnome, or if Gnome is totally borked, perhaps you could try re-installing Gnome. Once Gnome has re-installed, you could then try rebooting and changing the default session to Gnome before logging in. That should sort you out nicely.
Once you know Gnome is OK, you can then uninstall the KDE desktop if you want (or you could leave it in case you decided to go back to it and try it again at some point)!
Otherwise, worst case scenario: Back up all of your personal files and any major customisations you've made (I usually back up custom themes and screensavers I've made and a few other bits 'n bobs as well.) and then re-install Ubuntu. But obviously this is pretty much the last-ditch solution!
Personally I'm not a fan of KDE either! Far too many configuration options to customise the desktop to the way I like it. It also seemed really clunky, unintuitive and slow! I especially hate the main start menu...Yuk! I find Gnome far simpler and quicker to customise and use.
I had KDE, Gnome, Xfce and a few other lesser-known desktops installed on one of my Ubuntu boxes a while ago, in order to try and compare all of them and ultimately choose one to stick with. My final choice was Gnome, so I uninstalled all of the others and was right back to where I started! heh heh :)
Oh, one other thing of note strikes me: You may need to keep python 2.6 installed. I could be wrong but I think there are several critical parts of Gnome that depend on Python 2.6 being present. That could possibly be part of your problem, all of the python libraries used by Gnome will be python 2.x based! I think work is underway to port them over to python 3.x, but that could still be a long way off yet. Perhaps it will make it into Gnome 3 at some point, but who knows?!
Incidentally, you can have several different versions of python installed on the same machine, so you could install 3.x alongside 2.6 without any adverse effects on the system....At least, I think that's the case!
Cheers for now,
Jas.
