Sights which we can recommend from personal experience:

Painswick Rococo Garden
Ideal for Lunch/Afternoon Tea followed by a stroll through.

Sudeley Castle & Gardens and Berkeley Castle
Allow at least 4 hours. These are good castles to visit.

Nature in Art Museum
PICASSO in Gloucester?? Check it out.

For more ideas check the Gloucester Tourist Information website and http://www.quoakle.com/.