Gam -- pronounced 'gum' -- is the bija mantra of Ganesha; that is, the syllable that expresses his particular soul structure.
it means, chanting gam is connecting directly to Ganesha, to his soul, his energy structure. it's a brilliant practice.
the traditional Ganesh mantra is
om gam ganapathayay namaha
it's just been a Ganesha kind of day.