You are substantial enough to be attacked by mundane weapons, so you are substantial enough to be restrained/grappled
I understand that this seems illogical to grapple a gaseous form, but the primary problems are that it doesn't state immunity to those conditions and that the form itself is mechanically able to be interacted in a mundane manner via attacks.
There is nothing in gaseous form that states it makes you immune to either Grappled or Restrained conditions and spells do what they say and no more.
Because it doesn't state that it provides immunity to those conditions, then it doesn't provide immunity to those conditions.
Mundane interaction with the gaseous form
While the spell does state that worn/carried objects can't be interacted with, it does not say that the creature can't be interacted with. In fact it also states that it is susceptible to mundane damage:
The target has resistance to nonmagical damage...
While in the form of a misty cloud, the target can't talk or manipulate objects, and any objects it was carrying or holding can't be dropped, used, or otherwise interacted with. The target can't attack or cast spells.
If you can still be bludgeoned, pierced, or slashed while in gaseous form, then it's not unreasonable to also allow being grappled or restrained. There is clearly enough substantial matter left.
It's weird to think about, but while in gaseous form the target can still be grappled or restrained - just like it can be hurt by a mundane weapon.
What about the clause regarding openings/cracks?
To answer your concern quoted in the question regarding cracks/narrow openings, the problem isn't that it can use those - the problem is that you have no movement to use them.
If you are under the effects of Grappled/Restrained, then your speed is zero. If your speed is zero, then you can see those opening/cracks, but you can't utilize them to escape.
A grappled creature's speed becomes 0, and it can't benefit from any bonus to its speed.
A restrained creature's speed becomes 0, and it can't benefit from any bonus to its speed.
Without being able to move due to having their speed set to 0, they simply can't take advantage of those small openings.
It's not that they are paralyzed and can't do anything, it's just that if they are under those conditions, then they have no speed and therefore no movement to leave. The only way they can leave is by first removing the condition or by moving without using movement (misty step, dimension door, etc.)
Had the rules allowed for immunity to these conditions, they could have utilized the language found in the spell freedom of movement:
The target can also spend 5 feet of movement to automatically escape from nonmagical restraints, such as manacles or a creature that has it grappled.
Without the use of this, or any other, clause, the implication is that the normal rules apply.
There are monsters that are immune to the grappled condition. Many of them have similar 'properties' to that of gaseous form (air elemental, i'm looking at you), but each of them specifically state they are immune to the grappled condition.
It is that listed immunity which provides the immunity. Without it...they aren't.
As another example here, the gray ooze can fit with openings as narrow as 1 inch without squeezing. And it's an ooze. And this ooze can be grappled.
How can it escape?
The same way every other grappled creature can. They can either magically move themselves with an action/bonus action with spells like misty step or dimension door, or they can use their action to try and break the grapple with a contested ability check.
A grappled creature can use its action to escape. To do so, it must succeed on a Strength (Athletics) or Dexterity (Acrobatics) check contested by your Strength (Athletics) check.
Being able to pass through small openings doesn't give you a free grapple removal (if it did, it would say so as in the freedom of movement spell.) It enables you to pass through small openings when you actually have movement to pass through them.