Liquid Nitrogen..... you can find it in a spray can at a janitorial supply house. You spray it on the gum and it freezes the gum and you should be able to chip it off the leather. Janitors use it to remove gum from carpeting.
air duster will work fine can be purchase at wal-mart, k-mart or any pc place. Be sure not to spray on yourself with it, awhile back i was being dumb so i sprayed myself with it and it burned me and it was freezing cold.