Not necessarily. SSB is just regular SS with SSG transformation. Its not quite the next level of SSG. It's just combining 2 different transformations. Similar to the SSB Kiaoken, which isnt the next level of SSB its just two forms melded into one which amplifies the former form to a degree.
Edit: I may be wrong but that's how its described. Another example would be when Goten and Trunks did the fusion dance in base form but found out that if they did it as SSJ they would fuse into something stronger.
It is the next level of SSG. That's why they call it SSGSS in the literature. Its literally a Super Saiyan version of SSG.
They can skip to any form if they want. Goku can go from base to SSJ3 or SSJ2 to SSB or whatever. That's irrelevant.Basically combining something they already know with something new. Its why they can go straight to SSB without going SSG first.