place name, frankly
鄂 is a rare kanji used primarily as a place name. Its secondary meaning of 'frankly' is obscure and likely derived from its use in classical Chinese contexts.
鄂 combines 咢 (a phonetic component associated with surprise or frank speech) with ⻏ (a mound or city wall), suggesting a place name. The exact historical development is uncertain.
The left side 咢 looks like a mouth 口 over a base, and the right side ⻏ is a city wall. Imagine a place where people speak frankly at the city gate.
For ガク, picture a gak (invented sound like 'gawk') of surprise when someone speaks frankly at the city wall: gak -> ガク.