Yeah, if you can't use xhost as the user you logged in as, there's something up. Though if you can use X at all, you should have access to the display to be able to change its access rights...
Looking on Google, all reports of the GTK warning seem to be either (i)related to SSHing into another machine (the vast majority), or (ii)people unable to start X at all. As noted, if you can log into X, that user should be able to access that display, and... hmm.
Just to check, you are trying to run things on your own monitor and not on some other computer? If so, perhaps the $DISPLAY environment variable is set so it's looking somewhere else. Try "echo $DISPLAY" from a console to see if you're inadvertently trying to start your app on a remote PC. It should almost certainly be ":0.0", so if it's not, try
and see if you have any more success.
If that doesn't sort it out... can you run the standard X applications, and other downloaded ones? It could be that your Gtk toolkit isn't installed properly, though I probably won't be able to help you with that other than suggesting a
yum install gtk2
which even then is just a guess.
Hope one of these things sorts out your problem,