The description for the create or destroy water spell (specifically the Destroy Water effect) says:
Destroy Water. You destroy up to 10 gallons of water in an open container within range. Alternatively, you destroy fog in a 30-foot cube within range.
And the gaseous form spell description says:
You transform a willing creature you touch, along with everything it's wearing and carrying, into a misty cloud for the duration.
"A misty cloud" sounds a lot like fog to me. And the description for create or destroy water doesn't mention any restrictions regarding affecting magically fog.
So could you kill, damage, stun, or otherwise ruin someone's day by Destroying their gaseous form?