The GSX manual, page 98, explains all possible issues with passengers either missing or with missing textures, with all known solutions. In case of 3rd party liveries, the most likely cause is the name of the package not conforming to the proper naming pattern, which for the PMDG 777 is anything that starts with "pmdg-aircraft-77w", so it works for the base package and the "pmdg-aircraft-77w-liveries" where all official liveries are installed by default.
If you have an unofficial livery, be sure the package is named like this to be recognized, for example "pmdg-aircraft-77w-mylivery" or "pmdg-aircraft-77w-something-I-downloaded", they all work, as long their name starts with "pmdg-aircraft-77w"
I did try to uncheck and recheck on the installer GSX passenger box but when a do that the texture of the plane got only fuselage with no wing and i see the fuselage with no exterieur skin that i can see the seat like a bare plane.
I cannot reproduce this, please try uninstalling/reinstalling the airplane and try again. If that doesn't fix it, provide a link to the livery that has this problem.
