checking for GTK+ - version >= 2.4.0... no
*** Could not run GTK+ test program, checking why...
*** The test program failed to compile or link. See the file config.log for the
*** exact error that occured. This usually means GTK+ is incorrectly installed.
configure: error: GTK+ 2.4 or later isn't available, so Wireshark can't be compiled
Disable the 'profile' flag as per this bug. So the magic command is:
USE="-profile" emerge wireshark
Happy sniffing! ;]