To use the King James Version of the Holy bible on the Bible Gateway website, go to the home page and look for the search engine field at the top. To the right is a pull-down menu with options for English translations of the Bible, including four King James variations.
Some words used in the King James Version differ from more modern translations. For example, the word "thou" is used instead of "you." To search for the verse where Jesus appoints Peter as head of the church, use the term "thou art Peter" instead of "you are Peter." Bible Gateway does not find Matthew Chapter 18, Verse 16, when using the term "you are Peter."
To do a cross-referenced search, when you know the modern English of a certain Bible passage, do a search for the phrase in a modern translation such as the New International Version. Note the book, chapter and verse, then search the KJV by book. If you want to read "the Lord your God" in context, search for that phrase in the NIV and see it is in several books. In the New Testament, note it is in Matthew Chapter 4, Verse 10. You can then search the KJV by typing "Matthew Chapter 4 Verse 10" or just "Matthew 4:10" in the KJV search engine and read it in context. There you will see it says "the lord thy God."