Hi XsniferX,
I had a quick look on the OpenTherm Gateway. It has the option for a OpenTherm and/or on-off thermostat. But it mentions that running it with an on-off thermostat might be less effcient than with an Opentherm protocol thermostat.
This multizonethermostat does not include the OpenTherm protocol. You could use the on/off signal of the master to operate a relay which is connected to the on/off contacts. If you are thinking to use OpenTherm communication then some work has to be done 
To anwser your other questions:
so I would like to :
Be able to control heating per room (maybe also activate heat in all room and set the target setpoint once for all room. if one room gets warmer quicker than the other room close only the actuator for that room but leave the climate ON for other room)
Based on the temperature of a room if heating is needed open the actuator then activates the climate/burner. (The actuator take up to 2/3 min to get fully open or close, so I want the minimum duration of a heating session to be at lease 30to45min to preserve the boiler.)
If no heating is needed turn of the climate and then close the actuator.
It is possible to have two “heat:” for a thermostat? One for the actuator and one to send a command to the OTGW to activate the burner maybe?
This multizone thermostat allows indeed multiple room thermostats. For each room a thermostat is configured (satellite), each one reads a temperature sensor and controlling a NO/NC valve. A PID controller determines how long the valve should be open/closed.
An additional thermostat is configured acting as master controller. The master organizes when each room valves opens/closes (method depends on chosen config) and gives a signal to the heater to start and stop (based on the room valves opening).
Just for in case you are busy implementing it. Currently I’m updating a bit of code which might also affect the NO valves. I hope finish it within the next week(s).