Gabion walls as retaining walls?
A quick search for "gabion retaining walls" can find you lots of information.

It found me this useful site:
https://www.gabion1.com/gabion-retaining-wall-designs/

As to your questions:


3 feet is pretty short for a retaining wall, the site recommends a simple 2:1 ratio, so a 3' wall would need to be at least 18 inches thick.  They also recommend slanting the wall slightly into the slope (~10 degrees).
Drainage is important, but depends a lot on location, you can run perforated drain pipe behind the wall to collection points and backfill above and around the pipe with gravel.
The back of the wall should be lined with geotextile fabric, this should also cover your drainage gravel to prevent clogging the drain pipe.
Gauge size is going to depend on how wide your mesh spacing is.  Larger spacing is going to need thicker wires.  looking at premade gabion boxes selling for 3 foot walls looks like wire size varies from 9 gauge to 4 gauge.