At what altitude a ballon is no longer pushed by the wind?
In the range where a balloon can fly, there is always wind.

The NASA Echo 1A satellite was used to test the satellite triangulation. GPS works in some other way...

The GPS system used active satellites (non-pasive as Echo 1A) that constantly supplies their position, their time and additional coding information. So the satellite position should be known and corrected constantly. With a balloon constellation it will be very difficult.

Supposing that we can install all the equipment in a balloon, the top we could reach will be 53 km. The actual Navstar GPS constellation it is working at 20.200 km from our surface. So it means that we will need a lot of balloons in order to create a similar network, with a lot of ground stations to manage it.

Even if that point could be possible, there will be additional problems, as the areas with no islands or no continents when additional ground stations should be required to correct the signal and position.

The actual GPS satellite geometry was decided in order to reduce the number of satellites with the maximum cover.