outspokenly
咢 centers on speaking out boldly or being outspoken. The single given meaning is narrow, so treat it as a rare kanji for forthright speech.
咢 combines 吅 (two mouths) and 亏 (a pictograph of a breath or exhalation), suggesting the idea of speaking out or vocalizing strongly. The exact historical development is uncertain, but the component combination points to forthright speech.
Two mouths (吅) on top of a breath-like shape (亏) suggest people speaking out loudly and directly. Imagine two mouths arguing openly, with their breath visible below.
For ガク, picture a gak (invented sound like a sharp, outspoken retort) coming from the two mouths: gak -> ガク, as they speak out boldly.