From the FAQ, on page 4:
Q: Can the Scientist’s ability stop a Dreadnought’s bombardment
with the Graviton Negator technology?
A: Yes. The ability provided by Graviton Negator should be
treated the same as the War Sun's bombardment ability.
And from the rulebook, on page 31:
• Like the Dreadnought, a War Sun unit is allowed to
bombard planets. Unlike the Dreadnought, however, a
War Sun ignores the presence of a PDS unit's planetary
shield. Also, a War Sun may bombard a planet
during the Invasion Combat step of the Activation
Sequence, even if no friendly Ground Force units
have landed on the planet in an invasion attempt.
Therefore, if you have the Graviton Negator, would your Dreadnoughts also be able to bombard without ground forces in an invasion like a War sun?