With the old version 1.8.9.4 i could resolve the pb assigning right forward (the more forward) service door 1 to door with open/close checks as FSX Door#4 and right backward (the more backward) service door 2 to no door at this position. With the new version 1.9.0 it doesn't work! It is stopping to the message catering service is on their way again!
I don't know what I did to the configuration file but, I just tried asking for catering with the Dash 8, using the default configuration that comes with GSX, and catering completed correctly. I had a truck on the aft door, and a minivan on the front door, which is correct. Try to Reset the configuration to default, and "Restart Couatl".
However i have fsx installed too and after deicing for example my PMDG 737 at Chambéry France VFR i don't have the pushback, i have to reset position to access pushback option! I have desinstalled the 1.8.9.4 version completely before to install 1.9 so it mustn't be a problem of update files! However at LSGG Fsdreamteam it works! So it seems not working on all airports.
There was a problem if the pushback preference was set to "None", but this is already fixed, so you should get it automatically if you reinstall or just check for updates with Live Update.
Instead, there's another problem we just found, that if you select the "Customize this parking position" option to change anything, the pushback will not work anymore after you Apply the changes, so the solution is to select "Restart Couatl" AFTER you made a change to the customization page.