The minute someone uses an open-source GPS radio to build a cruise missile in their garage, and uses it for assassinations.

Given the current tech level available to hobbyists, this isn't that far fetched.

You don't need an open source GPS radio for that, just fly a bit slower. The upper limit is plenty fast for weapons, 1900 km/h isn't much of a limitation, neither is 59,000 ft of altitude.

If you can understand the equations and engineering needed to build a cruise missile the GPS equations will not daunt you. Getting the final approach to have a useful Circular Error Probable at anything low enough for an assassination would be more of a technical challenge than the coarse guidance. Unless you had someone shining a designator you’d need real-time machine vision. To say nothing of designing an airframe that can perform precision manoeuvres at speed without breaking up.