You are saying "impossible" because someone made a great website going over what is known and what is postulated. One I argue misinterprets two truths.
First, your argument on the energy in the gama is wildly wrong and shows you do not understand what you are reading (I know -$k). That one fuel source has excessive neutron bombardment is not the argument against the gama being the Grail of fusion. We could jump on redit and state that GIVEN MAN'S PRESENT KNOWLEDGE AND TECHNOLOGY we should move up the food chain to find the safest way just to release the damn thing. Nickel? Etc.
Secondly, the conventional intellectual mind breaks on the correlation between speed (toward light), time dilation, and potential. The physics twerk being described is your friend, if you can pay for it (energetically.) from the outside of acceleration, do not get lost on time, rather mass gain. Standing waves and constructive and destructive interference, and pulse modulations later, can a self contained counter weight be accelerated toward light in the controlled arbitrary displacement of mass (in this case the craft's center of gravity.)
Yes.
With the gama harvesting true fusion offers, the superior propulsion of mass acceleration toward light speed would change our everythings.