GPS isn't quite accurate/reliable enough. I know there are issues with shared scooters/bikes that are speed limited by GPS (I specifically heard this complaint about Neuron's scooters and bikes in Waterloo, Ontario).
I'd much prefer we design the roads to limit speeds.
Yes, though they're generally considered bad road design. I'd prefer continuous sidewalks (which are kind of speedbumps), bumpouts, reducing road width, chicanes, pavers on slow roads, etc.