What conditions cause the gear-up warning to sound on a Bombardier DASH-8?
According to smartcockpit.com, warning is given when the gear has not been extended and the aircraft is in the landing configuration. Specifically,

• All landing gear not down and locked, flaps selected to 15 or 35$^{\circ}$, and both power levers at any position less than 50% torque.
• Landing gear not down and locked, one or both power levers at or near FLT IDLE and airspeed below 130 kts.

The warning also sounds when the aircraft landing gear is up and terrain clearance is inadequate during the cruise and approach phase
of flight.

Image from Dash8-200/300 - Warning Systems from smartcockpit.com