Pittosporum gayanum is a shrub or small tree typically ranging from (1–)3–5 meters in height. The young growth and branchlets are densely covered in brown to reddish-brown woolly tomentum, although older parts may become glabrate over time. The leaves are simple, alternate, and typically large, with obovate to oblong–obovate blades measuring (4.5–)9–30 cm long and (1.5–)3–10 cm wide. Leaf apices are broadly acute to short-acuminate, and the bases attenuate. The upper surface of the leaf is glabrate with prominently impressed veins, while the lower surface is densely covered in brown to dark reddish woolly tomentum, particularly along raised veins, though this may become sparser with age. The blade is coriaceous and strongly rugose with entire, usually revolute margins and 7–20 pairs of lateral veins. Petioles are (0.6–)1–3 cm long and are often narrowly winged. Stipules are absent. The flowers are unisexual and borne in very densely tomentose inflorescences, usually in simple, axillary or cauline, corymbose clusters of 5–15 flowers. Peduncles measure 5–17 mm long, pedicels 2–9 mm, and bracts are subulate and 2–10 mm long. The calyx consists of five distinct sepals, 4–6.5 mm long, ovate, and densely brown tomentose. The corolla has five white petals, each measuring 14–24 mm in length. There are five stamens alternating with the petals; filaments are 10–11 mm long (ca. 6 mm in pistillate flowers), and anthers are 3–4.3 mm long (ca. 1 mm in pistillate flowers). The ovary is superior, borne on a short gynophore, 2-carpellate and 2–3-celled, with parietal placentation and numerous ovules arranged in two rows per placenta. The style is single, and the stigma is capitate. The fruits are capsules that are broadly ovoid to subcuboid–ovoid, 18–25 mm long, initially brown to reddish brown tomentose and becoming glabrate at maturity. The apex is broadly acute with a persistent style 4–6 mm long. The woody exocarp is 2.7–4.7 mm thick and may exhibit a median ridge or furrow. The fruit surface is rugose–tuberculate, and the troughs between convolutions are narrow and sometimes obscured. Capsules are typically filled with a seed mass of approximately 17 black to reddish-black seeds, each 6–7 mm long, irregularly angled, and compressed. It is found in open bogs, wet forests, and diverse mesic forests, between 640-1,520 meters.