Reactive windshear warnings (and recovery guidance) are generated by the flight guidance system including the IRS and other parameters such as IAS, not the GPWS
The GPWS generates the warning. The Flight Guidance systems provides guidance for the escape manoeuvre, which in reality, is just a max effort go-around!
GPWS provides immediate alerts, and look-ahead obstacle and terrain alerts for
potentially hazardous flight conditions involving imminent impact with the
obstacles and the ground.
GPWS immediate alerts and altitude voice annunciations during approach, are
based on a combination of RA, barometric altitude, IRS, airspeed, glideslope
deviation, and aircraft configuration.
GPWS alerts are provided for the following:
• altitude loss after takeoff or go-around
• excessive and severe descent rate
• excessive terrain closing rate
• unsafe terrain clearance when not in the landing configuration
• excessive deviation below an ILS glideslope
• windshear
• bank angle