Note that a gigaton = a billion tons and a kiloton = a thousand tons, so a gigaton = a million kilotons.
A reaper ship was harmed by a Mass Effect dreadnought's main guns, which is about 45 kilotons of TNT. Sorry to bump, but: A single shot from a medium turbolaser of a star destroyer has the destructive force of about 200 gigatons of TNT.
