I've already taken over GPS (not jam, emulate all 12 sattelites for a takeover) in a dummy load environment (large paint can grounded). It was with a ADALM-Pluto. Tx was only 5mW but I could easily build a multistage amplifier and filtered appropriately.
Why dont I? Its illegal and its ethically wrong, and its not my focus of study.
Generating the requisite signal is easy to do with opensource code and just $200 or so of equipment.
I bet the drone didn't use the military encrypted GPS because they didn't want encryption keys to fall into the wrong hands (they're global), or just because the effort to load the new GPS keys every week was too much, The attackers just jammed and then spoofed GPS, and by providing a very strong signal, any kalman filters designed to merge signals from inertial units would be fooled.
Why dont I? Its illegal and its ethically wrong, and its not my focus of study.