It doesn't "eliminate the possibility". At most, it reduces the probability, which is a very different thing.
Gamma ray busts are evidently fairly rare. One would have to happen fairly near to the Earth to have much effect.
Gamma rays would have to penetrate the atmosphere to have any effect on humans, and would have to penetrate quite a bit of water to wipe out "all" life on earth.
Passing through six thousand inches(500 feet) of air is enough to cut the level of gamma in half. To get from 30,000 feet up, to ground level, it would go through (very roughly) 60 reductions-by-half (and yes I know that the air at 30K is thinner than ground, but we're also talking about the whole atmosphere, so bear with me). 2^60 is about 10^18, a level of attenuation which is going to reduce the gamma level to a tiny amount.
Are you suggesting that the Institute of Creation Research doesn't know what it's talking about? Forsooth sir!
It doesn’t even REDUCE the possibility of life on other planets.
Apparently, with God all things are possible... except the creation of a life form that can survive gamma radiation. That’s right out.