GPS is a groundspeed indicator not an Airspeed indicator.
Verry simple said.
You fly into a headwind of 100 kts. with an airspeed of 100 kts. You GPS will indicate 0 kts. And your Airspeed indicator 100 Kts.
You turn your aircraft around and fly in that same headwind but now it is a tailwind. Your GPS indicates 200 kts. but your airspeed indicator still shows 100 kts.
Get your facts straight please.
Obviously never done a Unreliable Airspeed for real/sim. Had these 737s not continued to accelerate to almost Vmo at low level, the crews may well have been able to have more time to nut out what was going wrong; certainly, a lower speed would have given the stab less authority. Below 5000ft, the IAS will equal the GS, for all intents and purposes. You will not be doing 100IAS in a 737 and the wind won't be 100kts just after takeoff.
And on approach the GS is invaluable.