ruby .gsub replace/substitute a specific range of indexes

Ruby newbie here. I'm trying to create a method that 'corrects' titles of movies. The methods should capitalize properly, ignore capitalizing articles(a, an, the, of) unless at the beginning, and should correct properly even if the title given is all uppercase or jumbled. I get stuck when I want to specify a certain range of indexes in a string (1..100) to find articles and change them into lowercase. The first half of my code runs just fine, but the second half which modifies a range of indexes the .join is where i'm having trouble. If its possible to use .gsub's or 'if' statements after the ".join" that would be the best advice for my level of understanding. I hope I'm being clear. Any help/input is appreciated. Thank you

class Title
  attr_accessor :title
  def initialize(title)
    @title = title
  end

  def fix
    new_array = []
    @title.split.each do |word|
      new_array << "#{word}".capitalize
    end
    new_array.join(" ")
      new_array(1..100).gsub("Of","of").gsub("The","the").gsub("And","and")
    end
  end
end

alternately:

class Title
  attr_accessor :title
  def initialize(title)
    @title = title
  end

  def fix
    new_array = []
    @title.split.each do |word|
      new_array << "#{word}".capitalize
    end
    new_array.join(" ")
    if new_array(1..100) then
      new_array.gsub("Of","of").gsub("The","the").gsub("And","and")
    end
  end
end

here are the specs

describe "Title" do
  describe "fix" do
    it "capitalizes the first letter of each word" do
      expect( Title.new("the great gatsby").fix ).to eq("The Great Gatsby")
    end
    it "works for words with mixed cases" do
      expect( Title.new("liTTle reD Riding hOOD").fix ).to eq("Little Red Riding Hood")
    end
    it "downcases articles" do
      expect( Title.new("The lord of the rings").fix ).to eq("The Lord of the Rings")
      expect( Title.new("The sword And The stone").fix ).to eq("The Sword and the Stone")
      expect( Title.new("the portrait of a lady").fix ).to eq("The Portrait of a Lady")
    end
    it "works for strings with all uppercase characters" do
      expect( Title.new("THE SWORD AND THE STONE").fix ).to eq("The Sword and the Stone")
    end
  end
end