Not necessarily electronic, no. Electric would do, though I can imagine calling something pneumatic, hydraulic, or otherwise powered a device.
But "engine" as in motor is a bit too simple to be a "device" or "apparatus" (Gerät), I would say -- motors are more commonly part of a device (as with a washing machine, for example) but aren't devices of their own. Nor would I call a lamp a device, at least not a simple lightbulb. I guess for me at least, there has to be a minimum level of complexity.
On the other hand, Wikipedia says that even a plough is a Gerät.
Ah, and Duden reminds me of Geräteturnen - those Geräte are not only not electronic, but not even powered :)
(And for the "device" meaning of Gerät it defines it as "(movable) object which is used to work on something, do/accomplish something, or create something".)
tl;dr - I've never really thought about it :)