Using “ツ” would be like Apple using “
” because like how Apple doesn’t originate it’s meaning from “The Big Apple”, Grin doesn’t originate it’s meaning from a smile.
To put it another way, “grin” originated from Harry Potter lore, so it’s meaning and brand isn’t restricted but rather loosely coupled with various concepts (including a smile). Besides, the G incorporates a smile in its design intentionally (see this post Currency Symbol [ ɠ / ꞡ / ǥ / other ] #BS - #60 by shush)