As DM I have always some problems to apply the magic world of D&D onto the real world, which is the base of our common understanding of how things works. Especially, since I have experienced players who just wanted to kill kings to take over their kingdom, purely acting on impulse. I stumbled upon a new situation and I want to learn from your experience:
In my opinion, the spell gaseous form is very powerful for special-ops (by this I refer to rogues and assassins in particular). It allows to fly and, used at nightfall, it should generally provide a good amount of stealthiness as long as not used in very confined and lit spaces. Also, it provides the beneficiary of this spell the ability
to pass small holes, narrow openings, and even mere cracks,
Of course, it only allows slow movement (10 ft) and provides only resistance against common damage types, but the beneficiary can take all his potentially deadly equipment on its hour-long way. There might be a drawback; if the beneficiary is not the caster, the spell cannot be ended by the beneficiary, but that should be just a thing to be considered not a real problem here.
Consider a castle (or the home of a very wealthy merchant) which is well defended by regular means and maybe even some magical backup. An assassin who wants to follow his profession will certainly find a "mere crack" which can be used to enter the castle's building and even rooms (I don't think that any (land-based) door or window is air-tight).
This spell can be used from characters against anyone, as well as from assassins sent against the hero group (see Tyranny of Dragons, 2nd part for example). It could be used between hostile rulers - in fact, this could be nothing out of the ordinary: As gaseous form is a 3rd-level spell it even should be not a problem to find spell caster hirelings.
I don't see a realistic chance to protect a whole castle from such an intruder, do you? Certainly, spells like Alarm can 'protect' sleeping rooms, but they cannot prevent an attack (but, maybe a deadly success, depending in the fanaticism of the killer).
Do you consider a guard standing next to a bed all night as realistic?
Sure, while in gaseous form the intruder cannot interact with any object, but if that would be an issue I would send a special-ops team with potions of gaseous forms: whenever needed, a team member would end this effect, the remaining team proceeds.