All current aviation GNSS systems only use GPS (US system) as far as I'm aware. There are some GLONASS dongles such as Garmin GLO, but they're not IFR approved.

GPS and Galileo use the same frequency for their public service. Jamming one gets them both.

Glonass is a couple hundred megaherz away from GPS L1.

So there is no reason NOT to support Glonass.

Except the Russians run and control it and may consider opportunities to weaponize it offensively against unsuspecting Western users.

The best bet is to use as many positing systems as possible in order to prevent attacks narrowly targeting one positioning system.

The same can be said to NAVSTAR GPS. The only system which is not primarly built for military uses is Galileo.

Yes, that's what I am saying. Don't rely on one system. Using more also increases accuracy.