2GHz? Is that a clock frequency or the highest harmonic?
In the clock frequency case, it should be ok, but the eye will be getting a little fuzzy if you don't do everything to keep lengths down, in the harmonic case, meh, it will be fine.
I have a design doing 12Gb/s serial on FR4, and it does just fine, granted we had an SI analysis done on it, and had I had to specify the orientation of the glass mat.