nod approval
頷 centers on the physical gesture of nodding, especially to show agreement or approval. The meaning is tightly focused on that single action.
頷 combines 含 (contain, hold in mouth) with 頁 (head), suggesting a head movement that holds or contains meaning—a nod. The exact historical development is uncertain, but the phono-semantic structure is plausible.
The left side 含 looks like a mouth holding something, and the right side 頁 is a head. Picture a head with a mouth holding back words, so it just nods instead.
For ガン, imagine a stern general giving a single nod of approval: the general's 'Gan!' cues ガン as the head nods.