The book 'Drawdown' by Paul Hawken puts the number at about 15% of GHG by grazing animals. This is about 10 gigatons of methane being produced from grazing animals.

'Drawdown' puts the total figure around 50% because grazing land itself is added into the equation. This includes cutting down forests for grazing animals and lack of soil management.

I agree there is a problem with these numbers. They are generated from social data, how much is being produced on farms. The real numbers would be to empirically test the amount of methane in the atmosphere, or emitted into the air from farms. The problem is what type of data used; social data, or empirical data. Often when they say 10 gigatons, they estimate this from economic data, not scientific data.