Gregorian chant can't possibly date back "more than 2000 years." That would be before, or very shortly after, the birth of Christ. Christianity was well established in Rome and western Europe by the time there was any liturgical music that could be named for a Pope Gregory. 1500 years, at most, would be closer to the truth.

I can't sightread, not on my instrument and certainly not vocally. Wish I could. I know enough about standard notation to be able to slowly and laboriously figure out a melody from sheet music.

I do a little better with guitar tablature, if it's any good, because well-written tablature includes all the information provided by standard notation plus it shows where on the guitar to play each note. Problem is, there's a lot of very poorly assembled tab out there on the Web these days...