I also have a Samsung Epic 4G. Just installed Go Launcher EX and did not have the problem you describe. Try uninstalling Go Launcher, then re-install (from the native TouchWiz on the phone).
1. Press menu from a home screen
2. press settings > applications > manage applications > Go Launcher
3. Press uninstall > OK
4. Return to home screen (you will now be in TouchWiz or something like that)
5. If you don't have a Google Play icon on a home screen, open App Drawer to find it
6. Re-install Go Launcher from Google Play