While in this form, the target […] can enter and occupy the space of another creature. […] can pass through small holes, narrow openings, and even mere cracks, though it treats liquids as though they were solid surfaces.

This is one of the few transmutation and movement spells that don’t require you to end your turn or movement on an unoccupied 5ft square.

This should work. Nice! 😆

Hold on hold on…

Are there any rules about the mass of the Gaseous Form?

In the absence of a rule that specifies “reduced total mass” as part of the Magic effect, I would assume normal rules of reality apply.

As such, the same number of moles that previously comprised the Paladin and Dragon are now gaseous species at local temperature & pressure. That’s the magic, yaay!

But it’s the same number of protons, so it’s gonna be just as heavy. Even if you can squeeze all those molecules into a small-ish container, it’s gonna have the weight of the paladin, his armor, and his dragon.

It might have the same mass, but considering they gain a fly speed, their (effective) weight is zero.

If there are in a sealed container that wouldn’t matter, unless it flies using a magical force without an equal and opposite force, at which point you might as well just throw out conservation of mass.

Buoyancy. It has weight, but if it’s lighter than air, it still floats.

For it to be buoyant, density would have to be less than air. If we’re holding to conservation of mass, then the bottle would have to be significantly larger than the paladin, which doesn’t sound particularly stealthy.