The Booster Battle Pack contains everything you need for a two-player game of Magic. When you open it up, you'll find two semi-randomized "decks" of 22 cards. Each one will be associated with one of the Return to Ravnica guilds. Like this:
Yes, each one contains the relevant Guildgate.
You and your opponent decide which of you is playing which guild. Then you each open a Return to Ravnica booster (thoughtfully provided) and select up to five cards to add to your deck. You'll now each have a deck of up to 27 cards, and you can start playing immediately!