Aki has to sell you a potion to unlock that 6th gallery. To do this:
1. At the beginning of the game, you must choose that Aki's potion is better than Natalie's.
2. When Natalie and Aki debate about the best date for Bruno and Pibell, always side with Aki.
3. Buy the potion that Aki sells you.
4. Finish the date with Pibell at her house. Then, the CG will change.