The textbook thing to do is play a chord common to the two keys, called the pivot chord, then play the dominant of the next key.
Gm should be relatively easy, because
Cm is a chord (
Cm G7 Cm - something like that, elaborate any way you like, will establish
Cm as the first key. Then do
Cm D7 Gm to modulate to
Yes, to go back, using normal tonic/dominant harmony, the
Gm will become a
G7. Regarding the pivot chord of textbook modulation, there is nothing special to do to go from
Gm back to
Gm is diatonic to
Cm, so just playing
Gm provides the pivot. Just do
Gm G7 Cm and you're back to
How about using bVII7 to bVII42 but instead of resolving to III6 resolve to Gm? would this work?
Sure, you could use a deceptive progression. You can also use augmented sixth chord resolutions to modulate. You can just directly introduce the dominant of the next key, etc. etc. But those things by (textbook) definition are not smooth. They would be more like surprise moves that emphasize the chromatic color. The textbook "smooth" modulation tries to exploit what is diatonic and minimize the chromatic. In essence that is what make the smoothness.
This would mean making i into a major chord... not sure I like that sound, what else might work?
I think you mean the mode change of
Gm G7 is what you don't like and want to avoid. Just choose a different chord as the pivot. Technically,
Cm would be the most obvious common chord, so
Gm Cm G7 Cm will give you a diatonic pivot and separate
G7 by one chord.
The modal chords (as typical pivot chords) and the circle of fifths progression are another strategy. From
Gm you could get to
E♭ any number of ways and then just follow a circle of fifths progression into
Gm ... E♭ A♭ Dø7 G7 Cm.
Considering modal chords and how they can be used in modulation, the
♭VII in minor should be understood to have that strong modulating potential. Tonally it alters the leading tone, which weakens the tonality. It also introduces the subdominant degree of the subdominant key. In other words, it isn't merely an alteration of a sharp or flat, but a tonal alteration of that pitch's function from dominant in the one key to subdominant of another. Therein lies the modulation implications.
So, another option for smooth modulation, using a pivot chord, and avoiding the direct modal change of
Gm G7, can be had through
♭VII. Something like
Gm: i v6(min) stays diatonic to
Gm while also introducing the subdominant, tonal degree of
Cm, technically it isn't a pivot chord, but the new subdominant degree does a lot to begin shifting the tonality, continue as
Gm i v6 iv6 and we get to the
Cm pivot chord, then continue, we played the pivot chord so now we are free to play in
Cm, using the subdominant both makes for emphatic functional harmony in the new key and is consistent with how we departed from
Gm: i v6(min) iv6(Cm: i6) Cm: iiø7 (or iv) V7 i.
You could do many other things.