I'm a little confused by your post.
Casting gaseous form will let someone slip through the bars of the cage version of forcecage, but not the solid box version. Making it a reaction doesn't really help with that. Casting it as a reaction to being forcecaged, would only make the caster gaseous inside a force cage. They still wouldn't have a move to get them out of the area of effect until their turn anyway.
So if you are asking can gaseous form be used to exit a Forcecage, then yes, if it's the cage version.
If you are asking specifically for a reaction that eludes Forcecage then gaseous form won't work. You're better of making a reactive teleport that gets out of the area of effect, such as Misty Step as a reaction.
If you have PCs in your game who use Forcecage regularly and you specifically design monsters to circumvent their chosen spells, I think you'll find that your PCs will get a little annoyed with you.
If you have some BBEG that you're concerned about getting force caged, then try either the bait option (trick PCs into using force cage on a polymorphed rat made to look like the BBEG or similar). Alternatively, use the force cage against them. If they use the solid box form,, they can't hurt the BBEG either and an hour later it will be angry. Or give the BBEG Forcecage himself, use it defensively. If they trap him inside the cage version, he throws the solid cube version around himself.
EDIT: To clarify, I'm all for enemies with special abilities to surprise PCs with unknown capabilities and keep the PCs on their toes. However, if your only tweak is to nerf a PC ability so that it becomes pointless then that's a little unfair. Your problem then is with Forcecage, and if you don't know how to counteract it or plan for it in your adventure prep, then perhaps you should ban the spell and allow the PC to choose something new.