Choose gum made with natural chicle or other plant gum base, sold in paper or cardboard, or skip gum altogether. Bin it, never spit it.
Conventional gum base is a blend of synthetic polymers; at roughly 0.4 g of polymer per piece, a piece a day is around 120 g of plastic a year going into bins, drains and pavements. Brands certified under the Plastic Free Certification scheme run by A Plastic Planet and Control Union include gum makers.
The catch: natural gum loses flavour faster and costs more; check for Plastic Free certification rather than trusting a natural label.