Sound like you may have a weak spark. May not be a hot spark enough to fire on the gas. Try cleaning the points in the magneto. The gap is .015 for them. I usually use 400 grit or finer sandpaper. Works great.I have a Gravely walk behind machine i believe has the 6.6 horse power motor (large slanted spark plug and under "hood" air filter). The problem i am having is not firing with only gas. With Either it will pop out the exhaust but wont fire over. the starter may not be turning fast enough but im not sure. any pointer with getting this old beast running will be much appreciated!
You should post your question over on the Gravely/Ariens forum HERE. There's a whole lot of people that can help you out there!