TicTacToe game in Ruby
Gratulation! For  a beginner you made a good and clear game.

A first modified version:

class Board 
    attr_reader :board

    def initialize(player1, player2)
        @board = (1..9).to_a    #You can use also a range to initialize the array
        puts "Player 1 is %s, Player 2 is %s." % [player1.mark, player2.mark]
        @player_1 = player1
        @player_2 = player2
        show_board
    end

    def play

      until end_game?
          puts "Player 1 turn"
          put_marker(@player_1.mark)
          if ! end_game?
              puts "Player 2 turn"
              put_marker(@player_2.mark)
          end
        end
    end

    def show_board
        puts '-'*9          #I prefer some lines in the board
        @board.each_slice(3) { |x| #Good! You use slice.
            puts x.join(' | ') 
            puts '-'*9
          }
    end

    def get_place
      while
          place = gets.to_i - 1 #Use a return parameter
        #Check also if you get only values between 1 and 9 (no letters, no 0)
          if ! place.between?(0,9)
              puts "Invalid move, use 1..9. Please choose again!"
              next
          end
          if ! free_space?(place) 
              puts "Invalid move, Pos. %i is occupied. Please choose again!"
              next
            end
            break #stop loop, Value is ok
        end
      return place
    end

    def free_space?(place)
        @board[place].is_a? Integer
    end

    def put_marker(mark)
        @board[get_place] = mark
        show_board
    end

    def win?(player)
        ary = [[1, 2, 3], [4, 5, 6], [7, 8, 9], [1, 4, 7], [2, 5, 8], [3, 6, 9], [1, 5, 9], [3, 5, 7]]
        ary.any? do |array| 
            array.all? { |num| @board[num - 1] == player.mark }
        end
    end

    def draw_condition
        @board.all? { |num| num.is_a? String }
    end

    def end_game?
        if win?(@player_1)  #use other check. The comparison wit == true is not necessary.
            puts "Player 1 wins!"
            show_board  #Show end situation
            exit
        elsif win?(@player_2)
            puts "Player 2 wins!" 
            show_board  #Show end situation
            exit
        elsif draw_condition == true &amp;&amp; player1_win_condition == false &amp;&amp; player2_win_condition == false
            puts "It's a draw!"
            exit
        end
    end
end

class Player
    attr_accessor :mark

    def initialize(mark)
        @mark = mark
    end
end

new_board = Board.new(Player.new("X"), Player.new("O"))
new_board.play


I removed the reader place and the variable @place.
There is no reason why you need it.
It is only a temporary used variable, there is no need to store it in the class.
I replaced it with local variables and method return parameters.

The use of the return command is not necessary. I used it here to show explicit when I want to return a value.

I integrated the checks into get_place.
There is also a check for wrong values (like letters) and different messages.

I replaced player1_win_condition with a generic test with parameter.

I integrated the game in a method play. So it would be easier to start multiple games.

Most important: You used the fix constant 'X' and 'O' in your Game. But this data are also defined a parameter in the class Player. This can make problems if you replace X by x or something else.

What is missing:
* There is no check for similar Player.marks

What could be checked/Modified:


I don't like the end_game? in the actual version.
You could integrate it to play (player 1 can only win after his move, same for player 2. Then end_game? needs only to check for draw.
The board could be a simple array without initialization. nil would indicate free fields. This could make some checks easier. But with the initialization the board gets its initial values to be shown in show_board.  




Another version!

The first version had some code repetition. For each player the code was written again.

The new version avoids this repetion:

class Board 
        attr_reader :board

    def initialize(player1, player2)
        @board = (1..9).to_a    #You can use also a range to initialize the array
        @player_1 = player1
        @player_2 = player2
        puts "Player 1 is %s, Player 2 is %s." % [player1.mark, player2.mark]
        show_board
    end

    def play
      sequence = [@player_1,@player_2]
      until draw?
          puts "Player %s turn" % sequence.first.name
          put_marker(sequence.first.mark)
          if win?(sequence.first)
            puts "Player %s wins!" % sequence.first.name
            show_board  #Show end situation
            return 
          end
          sequence.rotate! #switch to next player
        end
        puts "It's a draw!" #end message if not left before, becaus eone player wins
    end

    def show_board
        puts '-'*9          #I prefer some lines in the board
        @board.each_slice(3) { |x| #Good! You use slice.
            puts x.join(' | ') 
            puts '-'*9
          }
    end

    def get_place
      while
          place = gets.to_i - 1 #Use a return parameter
        #Check also if you get only values between 1 and 9 (no letters, no 0)
          if ! place.between?(0,9)
              puts "Invalid move, use 1..9. Please choose again!"
              next
          end
          if ! free_space?(place) 
              puts "Invalid move, Pos. %i is occupied. Please choose again!"
              next
            end
            break #stop loop, Value is ok
        end
      return place
    end

    def free_space?(place)
        @board[place].is_a? Integer
    end

    def put_marker(mark)
        @board[get_place] = mark
        show_board
    end

    def win?(player)
      ary = [[1, 2, 3], [4, 5, 6], [7, 8, 9], [1, 4, 7], [2, 5, 8], [3, 6, 9], [1, 5, 9], [3, 5, 7]]
      ary.any? do |array| 
          array.all? { |num| @board[num - 1] == player.mark }
      end
    end

    def draw?
        @board.all? { |num| num.is_a? String }
    end

end

class Player
    attr_accessor :mark
    attr_accessor :name

    def initialize(name, mark)
        @name = name
        @mark = mark
    end
end

new_board = Board.new(Player.new('Fred', "X"), Player.new('Joe', "O"))
new_board.play


To identify the players I need also a name. Before you had the fix 'Player one'. You could also remove the text Player in the output (replace Player X turn with X turn).