speaking the truth
諤 centers on speaking the truth directly and openly, without holding back.
諤 combines 言 (speech) with 咢, which likely contributes the sound and may relate to a strong or direct vocal expression. The exact historical development of 咢 is uncertain.
The left side 言 is speech, and the right side 咢 looks like a mouth 口 with something forceful above it. Picture words coming out strongly and directly, speaking the truth without hesitation.
For ガク, imagine a person speaking the truth so boldly that they sound like a gong: gong -> ガク, and their words ring out clearly.