Gallium oxide, I believe, is an older substrate that I seem to recall helps high speed, high heat computing and is resistant to EMF.
But that’s reaching back to old days in my memory.
Gallium arsenide is widely used as a microwave frequency semiconductor.
Gallium nitride is used for blue wavelength laser diodes and LEDs, e.g. in Blu-ray DVD drives.
Gallium oxide appears to be used as a substrate for gallium nitride, but not as the active semiconductor material.