Here is the complete list of shortcuts:

Open Goldie Ruler
⌥ + ⌘ + G

Flip Horizontally
⌥ + ⇧ + ⌘ + H

Flip Horizontally
⌥ + ⇧ + ⌘ + V

Next Ruler Theme
⌥ + ⇧ + ⌘ + →

Previous Ruler Theme
⌥ + ⇧ + ⌘ + ←

Snap Rotate Ruler every 45°
Hold down shift as you rotate from the dot anchor point

Vertically and Horizontally Lock Ruler
Hold down shift as you drag from the middle white bar