I haven't reached 24 yet however from trying to buy the FGT in the Online Delership and reading online I think you can buy it when your reach B Spec level 24 even if your A Spec isn't there yet. Although if your Bob is level 24 and your B Spec is say level 23 then you will need still need that 1 level.
Don't quote me on it as it might be wrong but try and buy it just now and see the error it gives you which makes it sound like the above