As of right now it looks like we have several options for the Reaper:
1. Normal Reaper.
2. Brook gag? Like, wtf?
3. Hallucination
I stand by number 1 ngl. Brook is still with Robin and he wouldn't put up such a gag in a very important moment like this. Plus the fact that he is trying to actively hit him with a Scythe. Brook doesn't have one and he wouldn't dare to hit Zoro in a critical moment.
And Regarding 3, I highly because the drug is a medicine whose only focus is to make him recover. It doesn't have any effects that are focused on making him feel less pain, so it wouldn't really make sense for him to see such an hallucination because of a pill that heals you. Y'all could say that it's a hallucination because he is it the gates of death, and I could say that it's the first time that we see something like that in 1038 chapters.