You're correct that the first step is Shapeshifting. Any time a GURPS character has a normal form, but sometimes becomes a second, distinct form, you use the Shapeshifting (Alternate Form) advantage. If a character can turn into any form that they'd like, then they should have Shapeshifting (Morph).
I think the issue that you're running into with your intuition that Shapeshifting is "more focused on changing into different creatures using magic" is that GURPS doesn't really care about how you flavour your abilities, or what you look like in general, it only cares what advantages and disadvantages that physical form gives you. If you're an amorphous cloud that can take any shape at all, then GURPS as a system is only interested in how powerful the most powerful "shape" is. No matter what form you turn into, you need to enumerate what advantages and disadvantages are inherent to that form, so that you can know the point cost of the Morph advantage that lets you turn into that form.
You can always play fast and loose with specific point values to make the advantage easier to use in play, but at the very base, Morph lets you add a bunch of advantages and disadvantages that represent your new form.
As far as a specific form that lets you exist as a gas cloud, what you also need is a set of advantages that emulate the following abilities:
- Limited flight
- Is physically a cloud of diffuse particles
- Can pass through very small holes
The first two are easy. I would suggest Flight (Lighter Than Air)  and Injury Tolerance (Diffuse) . For the ability to flow through very small cracks, you're going to want to look at page 53 of Powers for the Infiltration enhancement to Diffuse. In short, it's a +40% enhancement that lets a Diffuse character fit through tiny cracks.
The full Swarm of Nanobots racial template would then be: Flight(Lighter than Air), Injury Tolerance(Diffuse, Infiltration) 
The full advantage that lets you morph into whatever shape you like as you describe would be: Shapeshifting (Morph, up to 84 points, Unlimited +50%) . You can save some points by saying that the cloud is your normal form, and that you can turn into any template with a cost of 84 or less: Shapeshifting (Morph, Unlimited +50%)  + Cloud Form  = .
If you instead built it as a set of Alternate Forms, it would be: Shapeshifting(Alternate Form, up to 84 points)