That's what I did
Then it will surely work.
A proposition can be left to the user the choice of keys
I don't fully understand you. Or, maybe, you don't fully understand how the GSX editor works. You DO have the choice, just not in GSX, but in the simulator Controls page.
That's because GSX does NOT try to read a key, it will read whatever key YOU have set in the simulator, to be assigned to the Slew keys.
That's why I said that, if it doesn't work, resetting the controls to default (ALL keyboard controls to default) should fix it, and why if it doesn't, it doesn't have anything to do with GSX, but just the sim not having anything assigned to the Slew, or another addon interfering.
I'll try to explain it again in different words: if you assign joystick buttons to the Slew events, GSX editor WILL react to them.