It took me awhile but I solved it. I am using FTX Iceland Demo. I read in the forum that GSX ignores FTX_OLC_EU8. This file contains Iceland airports.
GSX only reads the default APX41090 file. So when it loads first of all the jetways are elevated. Going to the GSX Customization screen from the ADD-ons menu in P3dV4.4 clicking on the little airplane lowers the jetways to ground level of 39.620m. Jetways however are positioned in the middle of the apron.
So I first copied and pasted the following files: FTX_OLC_EU8\01_APTBIKF.bgl and the file 02_APTBIKF_CVX.bgl. I then disabled them in the same folder. I then cut and pasted the original two files and placed them in my Addon Scenery folder.
Then I opened ADE175 and loaded the file 01_APTBIKF.bgl and simply saved it as BIKF_ADEP4_ADE making no changes. I then opened that file in ADE175 and went to Tools\Change Airport Reference Data. I changed the airport elevation to 52.12 which is the default elevation in APX41090.
I loaded P3Dv4.4. The jetways were all on the ground and positioned correctly at the terminal. In FTX Vector Configuration tool\Airport Elevation I disabled BIKF as the scenery was blocky.
Reloaded and the airport had the correct elevation and the jetways were correctly positioned.
This took me about 3-4 days of trial and error to figure this out.
I hope it may help someone else. I also read that other airports in Iceland may have the same issue. Well, I will probably just disable GSX if I run into issues as I pretty much fly around BIKF.
Off to fly......