The Gallery, also called the Codex, is a new feature added to the game with the 1.8 update. The Gallery is a visual guide to the heroes in Heroes of Dragon Age. The Gallery button is present in the center bottom portion of the Home screen.
Once you open the Gallery, you'll be able to view heroes and organize into smaller groups for easy perusing. Currently, you can choose to view heroes All together, by Group (role in the DA universe), Rarity, Faction, and Specialty (their attack type).
Heroes that you haven't obtained yet will appear in silhouette until you do.
Any hero that you've summoned or won in a challenge or event will appear in full view. You'll be able to view each Tier of hero, as long as you've had them at that Tier. Their base stats will also appear in a character information panel to the left of the character model.
If you summon a new hero or fight one in a PvP Battle that you haven't before, you'll get a small number alert on the Gallery button to let you know.
If a hero is highlighted in the Gallery, that indicates that you have a new image of them available. If a hero has a green check mark on them, it indicates that the hero is currently in your hero roster or inventory. And, if the hero has the orange and white squares in the corner of their picture, that means that they are currently assigned to one or more of your squads (see below for symbol examples).