Troubleshooting

1. open a terminal emulator and execute:

pgrep gcin

Can you see gcin's process ID, a number?
If yes, jump to step 3.

2. run gcin in a terminal emulator, and see its messages

If you see "IMOpenIM failed. Maybe another XIM server is running.", execute:

pkill gcin ; gcin &

3. check your environment variable, XMODIFIERS
Demonstration:

~$ pgrep gcin
7438
~$ vi /proc/7438/environ

7438 is gcin's process ID in the example. Gcin's default XMODIFIERS is "@im=gcin"

4. about GTK+ 2.x based applications

~$ gtk-query-immodules-2.0

See if you have:

"/usr/lib/gtk-2.0/immodules/im-gcin.so"
"gcin" "gcin Input Method" "gtk20" "/usr/share/locale" "*"

5. check your environment variable, GTK_IM_MODULE

It should be "gcin" or "xim".