Have you seen the Remote Sensing paper on "Testing the Applicability of Drone-Based Ground-Penetrating Radar for Archaeological Prospection (https://www.mdpi.com/2072-4292/17/9/1498)" by Linck et al.?
I'll definitely give it a read - the company I work for (Wessex Archaeology) did some flown GPR but those results aren't out yet. Not sure if they are planning to publish that too, but we're getting more and more flown mag and I think GPR will follow.