Down in the weeds, at approach speeds, if you did lose all your primary instruments you could use GPS groundspeed and altitude to get it on the ground. Obviously if you know your pitch and power settings, you will be using those as well, but the possible variations of weight, altitude and temperature means they are only 'ballpark'unless you have access to detailed performance data in flight. Older and smaller aircraft are unlikely to have anything other than approximate values.
If you were unfortunate enough to be faced with a limiting runway - e.g. very wet or slippery or minimum length - you'd want to be as precise as possible. Using wind reports from other aircraft and the tower, there is no reason why GPS groundspeed, adjusted for wind component, could not work in your favour.
I have run this as a LOFT simulator exercise, and once they gathered all the information, in almost every case the crew landed quite accurately.
Even in my little bugsmasher I keep an eye on GPS groundspeed and altitude down low just to confirm that the other indications are reasonable.
So, GPS is not totally useless for down low, at low speeds. Better than nothing.