type of thistle
莪 refers to a specific type of thistle, a prickly flowering plant. Its meaning is narrow and botanical, without broader abstract extensions.
莪 combines the grass radical 艹 with 我, which likely serves as a phonetic component. The character is used for a plant name, but the exact historical development is uncertain.
The grass radical 艹 on top of 我 suggests a plant. Picture a thistle with a prickly ego: 'I' (我) am a thistle, so don't touch me!
For ガ, imagine a gardener saying 'Ga!' in surprise when they touch the thistle: 'Ga!' -> ガ, and the thistle pricks them.
—