alder
榿 refers specifically to the alder tree, a type of deciduous tree in the birch family.
榿 is a phono-semantic compound: the left side 木 (tree) indicates the meaning, while the right side 豈 likely contributes the sound. The exact historical development is uncertain.
The tree radical 木 on the left tells you this is a type of tree. The right side 豈 looks like a complex shape, but just remember it's the alder tree.
For ガイ, imagine a guy planting an alder tree: guy -> ガイ, and he's holding a sapling with the 木 radical.