The warlock's Cloak of Flies eldritch invocation (XGtE, p. 56) says:
As a bonus action, you can surround yourself with a magical aura that looks like buzzing flies. The aura extends 5 feet from you in every direction, but not through total cover. [...] Any other creature that starts its turn in the aura takes poison damage equal to your Charisma modifier (minimum of 0 damage).
The gaseous form spell says:
You transform a willing creature you touch, along with everything it's wearing and carrying, into a misty cloud for the duration.
While in the form of a misty cloud, [...] The target can't attack or cast spells.
A warlock activates the Cloak of Flies invocation, optionally casts armor of Agathys, then casts gaseous form. Cloak of Flies is not a spell, does not have an attack roll, and persists until dismissed. Therefore, it should do damage to any other creature within 5 feet even when the warlock is a misty cloud.
Can the warlock expand the misty cloud from gaseous form so as to damage multiple targets? If so, how large an area could be covered?