This is a Sentient good (1st level of such). You must research the T4 good 1st. If it is your boosted sentient good, I would build and upgrade that manu (from planks) to produce it. If it is not boosted, you can trade for it by upgrading your trader.
Note: there are a few ways to tell if Tree Gum is your boosted good. I look at my Main Hall under Relics. Hint: if Steel is your boosted T1 good, then Tree Gum is your boosted T4 good (T1+1)