Who gets the credit...
...for naming them, I mean?
Archemaignites or ArchemaigKnights
Gnomen
(You could even use both!)
Archemaignites would probably get mispelled "Archa-May-Knights", supposedly derived from "Arcane Knights of Mayhem", eventually.
The "Allies" could still call them Panzergheist.
Commoners would probably call them "Metal-men", or something equally simple. The more educated/Nobles would probably call them "Automatons".
Personally, I'd stay away from "Mecha", including Mechanoids, Mechazoids, etc., etc., ad nauseum... but that's just me.