when I tried to activate GSX the standard key combination resulted in no action.
This is NOT Trial mode. Trial mode means that GSX will work only at default and FSDT airports and, if you tried to use it on another airport, it would display a menu saying you are in Trial mode, with a list of the Trial airport, and the option to go there. That's Trial mode.
If, instead, you just don't have the GSX menu anymore, it's not Trial mode: it's software not working.
Do you have both the Couatl and the GSX menu under the "Addons" menu of the sim ?