Aww poor baby, you got a lot of really great advice from members, bland diet until her tummy settles. Maybe just leave her on her old food, because of her age, as the quick food change might be too much for her system at her age. How long was she on the old food for? How was she on the old food, other than the gas? At 9 years old, any change than what she's been used to may cause an upset tummy, or the diarrhea. Sorry you guys are going through this, and I hope your baby feels better soon. Please keep us posted on how she is doing.
LEARN A LESSON FROM YOUR DOG, NO MATTER WHAT LIFE BRINGS YOU, KICK SOME GRASS OVER THAT AND MOVE ON.