This endemic, majestic aloe grows as individual, rounded, rosettes, about 1 m tall. The stems are usually short and seldom branched. The succulent leaves grow in rosettes of about 20 to 30 individual leaves, growing to about 600 mm long and 90 mm wide. The leaves are wider at the base and taper to a point. The upper surfaces of the leaves are grey-green, smooth, and flat or somewhat curved inwards. The bottom surfaces of the leaves are grey-green, smooth, flat or somewhat curved outwards. Young leaves have about 3–5 mm long, spiky teeth along the leaf margins, that are spread out, about 10–15 mm apart. The teeth are light brown, developing from a lighter coloured or white base. In adult plants, the leaves do not have teeth on their exterior. A few teeth may be observed in some adult plants, close to the tip of the leaf on the lower side.

Juvenile plants produce an individual inflorescence, however, as the plant matures, it produces an inflorescence that has 2 or more branches. Each inflorescence grows to about 360 mm long and 65 mm wide, appearing a darker shade of orange while in bud, and lighter in colour while in full bloom. The flowers of Aloe gerstneri are shades of orange. Each flower within the inflorescence is about 30 mm long, cylindrical, elongated and slender, progressively tapering to point. The pedicels of each flower grow to about 5 mm long. The bracts are 18 mm in length and 5 mm in width. Aloe gerstneri is known to flower in late summer, between February and March.

The smooth leaf surface and brown marginal teeth distinguish Aloe gerstneri from A. aculeata and A. reitzii. Aloe gerstneri is also comparable to A. petricola, which differs in flowering time and contains longer, bicoloured inflorescences.