create a board with all the cells you need

board.rb

class Board
  attr_reader :all_cells

  def initialize
    @all_cells = []
    (0..99).each do |x|
      (0..99).each do |y|
        @all_cells << Cell.new(x, y)
      end
    end
  end
end

find the neighbors of the ...it... cell

if cell.x_coord == x && cell.y_coord == x
(x - 1..x + 1).each do |i|
(y - 1..y + 1).each do |j|

          @another_arr << cell
        end
  end

Find the neighbors and sum their true states - 1.
The rules...

Every cell interacts with its eight neighbours, which are the cells that are horizontally, vertically, or diagonally adjacent. At each step in time, the following transitions occur:
Any live cell with fewer than two live neighbours dies, as if caused by under-population.
Any live cell with two or three live neighbours lives on to the next generation.
Any live cell with more than three live neighbours dies, as if by overcrowding.
Any dead cell with exactly three live neighbours becomes a live cell, as if by reproduction.