There was a problem in the GSX internal database for the Wilco 737-400, you can fix it by opening the GSX airplane editor ( from the Couatl menu, select "GSX - Edit aircraft data..." and change the two cargo doors entries with these:
[cargo1]
pos = 1.5 6.8 1.5 0.0
code = 2
name = Cargo Doors
uldcode = AUTO
[cargo2]
pos = 1.4 -8.95 1.75 0.0
code = 2
name = Cargo Doors
uldcode = AUTO