Considering my lacking but informed understanding of the fundamental requirements of life as it exists here on earth, I see a fundamental flaw with the idea that a galaxy sized organism could sustain the engine of life, protein folding.
There has to be some source of external energy that is fueling these organisms and galaxy sized anything make even the most massive stars in the galaxy look infitismally tiny in comparison.
Perhaps gravity of a super massive black hole could kick-start and spin some sort of electric turbine like biological organ or something to get things put into motion but considering life starts small such a scale would hit hurdles.
Of course this is assuming that life on distance planets all use the primarily carbon based deoxyribonucleic acid system of life.