Felix has suggested the idea that Greg can easily be interpreted as a trans allegory and I haven't stopped thinking about it ever since (exaggeration but I do think about it daily)
(transphobia in the comments)

in book 6 it's very significant that he feels horrible and hopeless because people don't understand he's a dog and a man,not one or the other,and not too much of one or the other
and they called him a misfit and were generally extremely hostile towards him

they refused to acknowledge the fact that they don't get to decide who he is

nonbinary dog man???

YES

nonbinary dog man my beloved

gotta love non-binary dog man!