While the Dimmer is set to work with the on/off switch the dimming with the switch is not possible the dimmer will fade between 0 and the previously set non 0 value, example if you set the dimmer before with the gateway 50% the switch will dimm the dimmer from 0 and 50% The dimm can be stopped during the fade if you trigger the switch again.
If the the on/off switch feature is needed, the parameter 1 have to be set to 1 (momentary switch) like this one click will be the start of the fade and the second will be the stop. The downside of this is that if you want to dimm directly to full you have to do on and off in less than a second with the switch to mimic the pres of a momentary switch.