To Quote your post
Unfortunately, because the spark gap the device employed (to tap radiant energy) caused serious radio and communications interference, the Federal Communications Commission ordered him to stop
By that logic every welding shop in the US should be shut down. An arc welder is a really, really BIG spark-gap device. And yet...we have welding shops. But WAIT...not only shops but portables some rather large. The ship builders of the Gulf Coast of Louisiana use portable welding units mounted on dually pickups that drive to the site to work, then drive home at night.
What you refer to are the conclusions c.q. proposals from Mark McKay, who investigated and collected all the evidence he could get his hands on. Quite a lot of it can be found at my site.
This is basically his best bet on what has probably happened. While this does not mean it is 100% guaranteed to be accurate, it does suggest the device employed radio frequency oscillations within the engine itself, which is totally unlike what you would do with a welder, whereby one is mainly interested in driving large currents trough the arc in order to melt metal.
Arc gaps can also be used in ** RF oscillators**, when used in their negative resistance area of operation. This is a technique dating back to the earlier 1900s. See the links in my article where I wrote:
This strongly suggests that this strange tube must have been a triggered spark gap, or better: triggered arc. Interesting detail is that arcs have a negative resistance area of operation, which can be used to build oscillators, which include radio transmitters. The Poulsen arc radio transmitter, for example, was widely used until the 1920s. If the coils within the engine were indeed part of the tank circuit of a spark gap (arc) oscillator, then it should be no surprise this technology has not been successfully replicated to date.
So, IMHO the conclusion that there cannot have been problems with the FCC is unwarranted, because the Gray engine probably utilized radio frequency oscillations, unlike what is done in welding units.
Please note that the theory proposed in this article does not address Gray's engine itself, but the power supply unit which could also be used stand-alone. So, the data about the engine itself is meant to provide a historical context rather than to analyze or describe the working principles employed in the engine itself.