The deuterocanonical books (sometimes collectively called the Apocrypha or apocryphal books) are texts considered canonical by some Christian traditions but by not others. These books are included in some Bibles and are omitted from others. Some Christians, particularly in the Protestant tradition, do not consider these books to be part of the biblical canon--that is, they don't consider them to be equal in authority to the other books of the Bible. Because of this, the deuterocanonical books do not appear in most Protestant Bibles. Other Christian traditions, including the Roman Catholic church, do consider the deuterocanonical books to be canon and include them in their Bibles.
For information on reading the apocryphal books on Bible Gateway, read this article: https://support.biblegateway.com/hc/en-us/articles/228180287