The Gulthias Tree in canon lore has no effective action of its own besides acting as the hive mind for the Blight infection. However, should you want to make the Gulthias tree encounter more exciting, you can add or replace the AC, hit-points, Dmg Immunity, and Experience listed above with a creature that you think best suits your campaign. The Tree Blight is a perfect monster to use. However, other options include a dryad, treant, or wood woad.