Leptomantis gauni is a small to medium-sized tree frog of lowland and hilly rainforest. 

A spur above the eye, a spur at the heel, and a white spot are typically present. Variation may exist in these features. The snout is short. Webbing on hand is not fully developed. Size of adults: up to 30 mm in males, 38 mm snout-vent length in females. 

This species can be seen during the night sitting on overhanging vegetation at streams. It prefers relatively large streams (at least several meters wide) with clear water and rocky bottom. 

Tadpoles hatch from small foam nests attached to overhanging vegetation. Tadpoles are of the clasping type that use their cup-like oral sucker to cling to rocks and hold position in the current. However, this is not a tadpole adapted for strong currents. It is rather found in riffles among rocks in moderate current (non-foaming part of rocky streams).