"In 2015, before we released to users, the Brave browser actually was built on the Gecko engine. However, early on our engineering team realized that Gecko lacked important product features, led to more Web compatibility (webcompat) issues, and overall had poorer performance." (1)
They did use Gecko as the base originally:
"In 2015, before we released to users, the Brave browser actually was built on the Gecko engine. However, early on our engineering team realized that Gecko lacked important product features, led to more Web compatibility (webcompat) issues, and overall had poorer performance." (1)
(1) https://brave.com/faq/#chromium-gecko