If you customize GSX to ignore a door, it will be ignored, you must have done something wrong, probably another door is still not set to be ignored.
In any case, the CS777 is already internally supported, at least the 200 version.
If you use the 200 version, it shouldn't require any customization and, if it's asking for "Exit 2" which is a generic code, it seems it hasn't been recognized, this will happen if you renamed the airplane folder under Simobjects\Airplane.
If you use the 300 version, is not included with the GSX database but, assuming it use the same variables, one way to customize it correctly without having to Ignore a door (which must work), is to load the 200 version, customize something not important (such as moving a door 1-2 cm) and than take the GSX.INI that will be created into %APPDATA%\Virtuali\Airplanes, and move it into the ROOT folder under FSX\Simobjects\Airplanes for the 300 version.
Then, load the 300 version, and this will load the 200 customization with the XML variables and custom names, so you'll only need to adjust their position for the different 300 size.