What happens if a creature in gaseous form dismisses the spell while inside another creature's lungs? I assume it doesn't just make their insides explode, as that would be too powerful, but I'm not sure what should happen (perhaps the target exhale the person before they completely reform, but still takes some con damage due to their lungs being injured.)
- Most spells can't be dismissed in Pathfinder but the discussion could still happen at the end of the duration of the spell -
If area, in this case, the lung of the monster, is big enough to accommodate the creature on which Gaseous Form was cast once it is back to being corporeal, I'd say the monster would take some damage from the foreign body in its lung. If the creature inside has enough room to move and attack and decides to do so, the damage to the monster would be great. Maybe I'd rule every attack is a critical threat, only needing to be confirmed and succeeding on the attack would be easy.
If however the space isn't big enough to accommodate the creature back to being corporeal, the creature would take a lot of damage as it is pushed towards a space that can accommodate it, just as if the spell had ended while it was going through a narrow pipe going thought a wall, partly in the first room, partly in the pipe and partly in the second room. From the level of damage, I'd find inspiration in other spells, such as those teleportation ones that have a clause about ending up inside solid material. The monster would take the same damage as when the creature solidified in its lung.