Something like this: [<10]0.0" GHz";0" MHz" will display any number <10 as "x.x GHz" and anything >10 will be displayed as "xxx MHz"

You didn't say what to do with values between 10 and 100, so I left those to be formatted as "xx MHz" but you could add another condition to do something different with those values.

This works for a limited number of categories (up to three or four, I think). Play around with it and see if it helps.