If you want to play this game online in the future, better upgrade... (Though I think most people here already have more storage - just shows how much of a weird product the 12gb slim model is).
How to Upgrade your PlayStation®3 12GB System Storage for Grand Theft Auto V
As the world of Grand Theft Auto Online continues to expand and evolve through ongoing updates, storage limitations of the PlayStation®3 12GB hardware will require owners to purchase additional storage to accommodate GTAV Title Updates after 1.16 (released on 8/19/14). All updates and content must remain installed on the system in order to maintain compatibility for all players within GTA Online.
For details on how to upgrade the hardware storage on your PlayStation®3 12GB System, visit:
PlayStation®3 12GB PS3 Information
If not enough space is available on the system, hardware limitations will prevent players from accessing GTA Online. GTAV Story Mode will still be accessible by playing the game without being connected to PlayStation®Network, and proceeding without additional updates.