How powerful(in terms of output watts)can current Gunn diodes get?
Basically no,

The Gunn diode is also known as a transferred electron device and relies on scattering to populate a tertiary band. The up shot is that this device needs to be biased to operate in it's negative resistance range. A lot of power can go into this biasing and indeed Gunn diodes are often heavily heat-sinked to be able to handle the power reliably at low oscillation power.

In short these devices are used to create oscillation, the power amplification step is seperate.