Natural selection doesn't act on a long-term basis - any inherited gene that is beneficial only with many generations in between the benefited generations is likely to be lost, and transpermia is likely to be a rare event. I'm with Nick that it's much more likely to be dumb luck than a selected survival trait.

My speculation is that life coudn't originate on a gas giant given the thousands of years that are likely required and the speed of atmospheric circulation, but life might be able to survive in a gas giant atmosphere if reproduction and diffusion is faster than atmospheric circulation into the ultra-hot regions. So all you need is an Enceladus-like satellite to start life (or transpermia from an earthlike planet to an Enceladus), have some micro-organisms freeze in the ice, and then have an impact transfer the ice to the gas giant.