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?

No. A "misty cloud" is not "fog".

In this Sage Advice tweet, Jeremy Crawford, one of the lead designers and rules arbiter for D&D 5e, states:

Create or destroy water—the spell can destroy a 30-foot cube of fog. A vampire's mist form is not fog or any other weather effect.

And also states here, as a follow up to the above linked question:

Create or destroy water—the spell can destroy a 30-foot cube of fog. Fog cloud creates fog. CorDW can destroy a 30-foot cube of it.

A vampire's Misty Form ability has similar wording to gaseous form in that it does not state they transform into fog.

Therefore, create or destroy water has no effect on a creature in gaseous form.

Fog is fog.

A creature is a creature: even if it looks like fog, it isn’t fog.

Create or Destroy Water needs a valid target, either water in an open container or fog - it has neither so it has no effect.

This sounds like it would entirely depend on how the GM interprets the Gaseous Form spell and if they decide that the misty cloud is formed of water or it's still the body but in a more vaporous form. I would say the latter since Destroy Water specifically says 'fog' and Gaseous Form says 'misty cloud'.

Misty can be used as a description of appearance, so the cloud could well be dust for all we know, while fog is pretty easily defined. RAW would be "transformed character is not fog, therefore spell does not work". It's the same as how that spell would affect a water elemental; is an elemental form an "open container"?

I'd be inclined to reward the players for thinking outside the box, and deal force damage as though it were a standard custom level 1 spell; 2d10 damage, with a CON save for half.