libgtk-1.2 is very old and unmaintained software, and it has been removed from recent versions of Ubuntu and Debian. Read more here:
Unfortunately, lots of old games and other apps still depend on it. If you must, install it with:
sudo apt-add-repository ppa:adamkoczur/gtk1.2 sudo apt-get update sudo apt-get install libgtk1.2
I needed this to install Darwinia, it seems it is also needed for Uplink and other games.