We are not ‘adding to the gospel’. You do understand, do you not, that the whole “Holy Spirit who will lead you to all truth’ was all about taking the oral, then written, traditions and explaining them further, right?
Furthermore, most Protestants do not accept John 6 as literal regarding the Eucharist being Christ’s Bread and Body, despite the fact that He actually says those words. So, ahem, it appears as though people felt that the Spirit directed them to understand (despite 1500 years’ worth of teaching otherwise) to ‘interpret the words as only figurative’.
It appears you’re perfectly fine with allowing the Spirit to ‘interpret’ even when it ‘interprets’ that words in Scripture don’t mean what they say. . .
But somehow we Catholics are not permitted to allow the Spirit to interpret words in the Bible to mean what they say and how that translates into actions by priests and people?