I know that at times, one has needed a $HOME/.scim file and had to add the hotkey thingie in there.
I have a page on this, though it doesn't really cover using the Gnome gui way of doing it. (Actually, the page is listed, with an incorrect URL, in the scim README, but it's rather dated, and as I don't use the GUI tools, I never really covered them.)
http://home.roadrunner.com/~computer...u/jpninpt.html
You might have to choose something from one of the Gnome preferences menus about input.
Just for fun though, try something like
XMODIFIERS='@im=SCIM' LC_CTYPE=en_US.UTF-8 GTK_IM_MODULE="scim" gnome-terminal
and see if it works.