Hi Manuel,
I don't know about the GoFlight AP product, but be aware that the ATR does not have an autothrottle. Thus airspeed is pitch controlled for a given throttle position.
If you want a higher IAS than possible in level flight with the current throttle setting, you will start to descend!
The other way around, if you set a flight level, but leave your throttle near idle, you are bound to pitch up too much, and eventually stall.
So I think (but as said, I do not "know") you need to somehow exclude throttle control from this AP product, or you might experience some unpredictable results.
Safe landings!
Eric-Jan