You know, I agree with you that you can’t just add your own “gospel” to the Bible and then call yourself Christian. However, that verse you cite is a pet peeve of mine because it’s so often misused in these circumstances. It doesn’t say anything about adding to the Bible, because the “book of prophecy” it is referring to is only the Revelation of John, not the Old or New Testaments. The New Testament wasn’t even collected as one volume at that point, so reading the verse in that context is nonsensical.
We are on the same side on this one so this is not a nasty challenge. God has admonished against adding or taking away from his word throughout the OT & the NT. I think it is a mistake to think that the revelation from Jesus to John was solely for the book of revelation and not a reiteration of a constant theme.
You COULD read it so narrowly as to apply only to the book of revelation but IMO it makes much more sense to apply it to the NT as a whole.
Galatians 1: 6-12 (specifically verse 8 and 9 )
8 “But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed.”
9 “As we said before, so say I now again, if any man preach any other gospel unto you than that ye have received, let him be accursed.”
* handles islam and mormonism