deb
http://ftp.debian.org/debian sid main contrib non-free
This one is official Debian repo. You already had it - probably without "contrib non-free" part, but it doesn't matter, as python-gtk2 for sure isn't non-free package :P
"Clever use of apt-get/aptitude" is needed only when something goes wrong with dependencies and you really want to switch between grip and main. In your case it doesn't matter, so just ignore it :P
If I were you I would just run aptitude and play within its GUI. On my Kindle I didn't have any problem with installing gajim, so there must be some reason why it doesn't work on yours (maybe I forgot about something I did before? I don't know...)