Originally posted by 753...which isn't nearly the same as using his personal power to destroy both universes.
IIRC G was being kept malnourished and sedated by thanos/annihilus tech, but he was being used to devour planets, however, they kept him from accessing that energy and the PC he metabolized from it. it was stored in canisters in the ship I believe. the plan was to accumulate a big enough meal and then rig his body to detonate, which would cause the extinction event that would leave annihilus as the sole survivor in the cosmos. I'll look for scans later
for instance, the US millitary started out with a single nuclear weapon -- which was enough to decimate a city, but that's about it. over the years, however, they've stockpiled enough nuclear weapons to destroy most everyone/thing on the planet. you see, they may have manufactured all the weapons, but that doesn't change the fact that the original weapon could only destroy a single city.
that said, if galactus was capable of storing -and not using- all the bio-energy he absorbs, then I have no doubt he could preform such a grand feat... but as it stands, he cannot. the energy he absorbs metabolizes FAR too quickly.