Can a person who casts Gaseous Form on themselves choose to dismiss it early?
From the PRD:
"(D) Dismissible: If the duration line ends with "(D)," you can dismiss the spell at will. You must be within range of the spell's effect and must speak words of dismissal, which are usually a modified form of the spell's verbal component. If the spell has no verbal component, you can dismiss the effect with a gesture."
Gaseous Form is listed as a Dismissible spell. And since it has no verbal component, you arguably need to make a gesture to dismiss it.
But once you are under the effect of Gaseous Form, are you able to make gestures? Gaseous Form notes that you "can't attack or cast spells with verbal, somatic, material, or focus components while in gaseous form." So if you can't make somatic actions, can you gesture?
"You must be within range of the spell's effect and must speak words of dismissal, which are usually a modified form of the spell's verbal component. If the spell has no verbal component, you can dismiss the effect with a gesture."
So to dismiss a verbal spell, the words you speak are a modified form of the verbal component. So to dismiss a somatic spell, the gestures you make would likely be a modified form of the somatic component. And since you can't make somatic actions at all in gaseous form, you can't make modified versions of them either.
The shape of your form isn't mentioned in Pathfinder, however, 3.5e noted: "A gaseous creature can move about and do the things that a cloud of gas can conceivably do". I don't see a cloud of gas being able to make gestures.