The doors were open and the trucks were parked and raised up to them.
The doors might have been opened, but if the plane is not correctly configured to recognize them, it won't detect they are. Some 3rd party airplanes are included in the GSX internal database, but if you are trying to use an unsupported airplane, since you only said "a 777-200", it's difficult to know if your problem that you tried to use an unsupported airplane without configuring it first, or you tried to re-configure an internally-supported plane and made some kind of mistake doing it.
The GSX airplane configuration page will tell you if GSX is using its internal database, or a custom customization, or it's lacking one.
Is there a pdf file to download or a book I can buy that will show me how to edit the plane's cfg file (whatever one I need to change) which doors open etc?
The GSX manual, of course, there's a complete section on airplane customization.
The manual can be accessed in 3 different ways:
1) From the Addon Manager menu inside FSX, selecting GSX and pressing the “Open Manual” button
OR
2) From the Windows Start button, under the FSDreamteam program group, there’s a link to the manual
OR
3) By browsing your FSX\fsdreamteam\Couatl\GSX folder, the PDF manual is there.