The airspeed unreliable memory items and checklist were developed by Boeing from recommendations / guidelines published by Flight Safety Foundation.
These guidelines included,
Procedures should provide information on altitude and power settings that enable crew to maintain the aircrafts flight parameters within normal limits during flight with unreliable airspeed events for all phases of flight.
Procedures should address the availability and use of independent alternate sources of airspeed information (e.g. GPS, inertial, angle of attack etc.)
Procedures should include memory items for critical immediate action steps
The recommendations also provided an Airspeed Unreliable Generic Template,
1. Disconnect Automation - Rationale: automation may be reacting to airspeed indications that may not be correct, so it must be disconnected
2. Stabilse the aircraft with the provided pitch attitude and thrust - Rationale: setting memorised pitch attitude and thrust settings stabilise the aircraft in climb or cruise as applicable. if in a descent, the aircraft should be levelled off and the cruise setting used.
There in ends the recommended memory items, reference items of the checklist then direct the amongst other items to set an applicable attitude and thrust. Boeing aircraft this information is found in the QRH PI section.
In summary
The initial pitch and power settings of the memory items on a Boeing aircraft are designed to keep the aircraft "safe" until such time as the accurate Pitch and Power setting s can be extracted from the QRH
PI section.
Regarding GPS ground speed, the elephant in the room is GPS jamming, it is becoming more and more prevalent in particular regions of the world so much so that IATA has recently issued an operator alert on the subject.