Final answer:
The Gnetales genus exclusive to the Namib Desert is Welwitschia, a long-lived plant with unique adaptations for desert survival.
Step-by-step explanation:
The Gnetales genus that can only be found in the Namib Desert is Welwitschia. This rare plant is not just unique to the Namib Desert within the arid landscapes of Namibia and Angola, but it is also the only extant species of its kind. The bizarre, long-lived Welwitschia is an especially adapted gymnosperm, consisting of just two leaves that grow continuously over potentially thousands of years.
Welwitschia mirabilis has adapted to survive in the harsh desert environment by having deep-rooted systems which can tap into underground water sources. Its immense longevity and the very fact that it is the sole species of its order make it a distinct entity within the Gnetales and a subject of great scientific interest.