If the fish has stopped eating, that's usually a bad sign. It can be difficult to diagnose fish, unfortunately. The spot on the tail could be bacterial fin rot. However, if the fish stopped eating I would guess that the tail-fin rot is not necessarily your primary problem. The fish could be stressed by something else and that stress caused the lack of appetite, change in behavior, and now the fin rot. Your fish may have a bacterial infection, particularly if the gills are getting redder. Guppies also are noted to be frequent carriers of some parasites and those can be really hard to diagnose. Honestly, this is kind of a shot in the dark.
It's not a fun recommendation, but if the fish gets sicker, it may be wisest to remove the fish from the tank and quarantine it (if you can) or euthanize the fish. Financially, it's not always the best decision to spend $20+ treating with meds on a small fish that is so sick. Particularly with vague, but advanced(?), symptoms. Before anyone flames me for this statement, I'm not
advocating euthanasia, just saying there may be a cost/benefit assessment to be made, particularly if this is the only fish in the tank(?) and your funds are tight. I realize there are emotional attachments - I spent a ton of $$ treating, and saving via the meds, my angelfish. It wasn't cost-effective, but I didn't care
Sorry I don't have a more definitive answer for you. Hopefully someone else will comment also. Welcome to the forum in any case! Unfortunately, all of us have lost fish. It can be really frustrating!