We had radio amplifiers that could transmit powerful signals and those were pushed to microwaves. We had lasers that were originally natural transitions in solids and gasses but now tunable lasers go from deep in the infrared to near UV at least, and with wigglers, undulators and free electron lasers we can go from UV to X-rays.
Above X-rays is a semi-infinite range of photon energies that have yet to qualify, so gamma rays would not count as a "hole".
I am certain I'm missing other technologies as well.
What I'd like to know is which wavelength range was the last "hole" in the spectrum where there was no technology to make amplified narrow-band power?
Question: What was the last "hole" in the electromagnetic spectrum where amplified, tunable, narrow-band signals couldn't be produced artificially?
I'm guessing it's somewhere between a millimeter and a micron in wavelength, but I could be way off.
note: I say "tunable" to indicate that you can center your narrow band anywhere within a wide range so that there is continuous coverage. Radio, optical lasers, synchrotron radiation-based sources are all tunable, while the old gas and solid state lasers had fixed frequencies and big gaps in between.