What you need is a compatible solvent, and I know for sure that turpentine is just the ticket. It’s made from coniferous trees, so it’s ideal for the job. Moisten the area with turpentine, let it sit for five minutes, then wipe the area clean with a rag. It might take several wetting and wiping cycles to get all of the gum off your deck. Sometimes hardened sap also needs to be scraped, too, though the turpentine will make it soft and easier to remove.