when i open the cargo doors and select the option request for boarding in GSX the Cargo loader vehicles arrive but both of them arrives on the tail side cargo door...
No, they don't. Surely not with the default configuration that comes with GSX.
The reason why they go on the same door on your system, must be you have tried to configure it, most likely you tried to exchange the doors to have jetways going on the 2nd door (as was suggest here on some thread), and made some kind of mistake editing it.
It might also possible that you downloaded a GSX.CFG file made by someone else, and that has an error, or it was made before GSX supported the PMDG 777 but, since user-made files always take precedence, is causing this issue.
The Reset option in the GSX airplane configuration editor will restore default config that comes with GSX.
Note that, if you placed a downloaded GSX.CFG file in the FSX\Simobject\Airplane folder (instead under %APPDATA%\Virtuali\GSX), this won't be reset by the above option, because is supposed to be a airplane developer provided config, so the reset won't touch it, which means you must remove it manually.
AND, restart Couatl from its menu after doing that, to have the default configuration being read again.
