Gangi (Sk.) A renowned Sorcerer in the time of Kâsyapa Buddha (a predecessor of Gautama). Gangi was regarded as an incarnation of Apalâla, the Nâga (Serpent), the guardian Spirit of the Sources of Subhavastu, a river in Udyâna. Apalâla is said to have been converted by Gautama Buddha, to the good Law, and become an Arhat. The allegory of the name is comprehensible : all the Adepts and Initiates were called nâgas, “ Serpents of Wisdom”.
Source: H.P.Blavatsky - The Theosophical Glossary