There are many gospels that are not included in the Bible. The Bible was never "one book". The process of putting together the Bible is known as "canonization" and is well documented. Back in the 100s and 200s, books were written on "scrolls" - long parchments wrapped around a central core. These scrolls could be 35' or more in length. After 200, works were put into Codexes, individual pages bound together, for ease of reading. Which of these scrolls and codexes were included in the "official Bible" varies from group to group. For example, many religions believe in the "Old Testament" - but the Jewish version has the books in a different order than the Protestant version. The Eastern Orthodox religion includes books that the others do not.
The Samaritans only recognize five books in their Bible. The Ethiopian Orthodox church has 81 books in its Bible. The Syrians have 22 books in their Bible, while the Roman Catholics and Protestants have 27 books.
Dan Brown's error in this case is that there are NOT 80 non-canonical gospels! Sure, we just discovered a few more gospels in the last century which is rather amazing after 2000 years. But most researchers agree on a total gospel count of about 35.
Not only are there different books, but there are different translations. The various books were written in "ancient tongues" that even now we do not necessarily fully understand the nuances of the languages. So when we then turn these books into English or French or other modern languages, we can easily mis-interpret or put a different shading of meaning on what they say. The books of the Old Testament were written in Hebrew and Aramaic. The New Testament books were done in Greek. And we don't even have the "original" of these books - we have copies made by monks. So we are relying on those monks to have copied correctly.
For example, in Greek, the word pistis can either mean "faith" or "believe". So depending on who translated it, that word might show up differently in English. The Greek word dikaiosune can either mean "just" or "righteous". Again, whoever translates that word must choose how to be turn it into English to make the sentence read well. The Greek word "logos" can mean: speech, message, account, statement, saying, phrase, word. That's a lot of meanings to choose from, when making an English version!
On the other hand, sometimes translators try to add meaning by choosing words not necessarily called for. The Greek word for "messenger", for example, is sometimes turned into "angel" when written in English. Maybe the messenger WAS an angel - but that's not what the book says. It simply says a MESSENGER came.
Also, many of the books were written in poetry style, or to go with the tunes of popular songs of the day to make them easy to remember. There were metaphors used that work in the original language. So translators have to try to translate the meaning *and* the rhythm of the words which is really hard. Think of the classic saying "how much wood could a woodchuck chuck if a woodchuck could chuck wood?" It is cool in English, but if you turn it into French, the French would wonder why the phrase is interesting. It would lose its rhythm and similar-sounding words.
In the same way, at one point John the Baptist says "God is able to raise up children for Abraham from these stones!" That seems pretty meaningless to us in English. But in Aramaic, the word for children is "banim". The word for stones is "abanim". He was making a play on words that his audience would easily remember.
Finally, as an interesting point, Christ is NOT the last name of Jesus. Christ is simply the Greek word for "Messiah". When the Bible's books were written in Greek, they were simply saying "messiah" when they said Christ. However, when that same word "Christ" was brought over into English instead of properly being translated as "messiah" in our language, people got confused.