Tell me how to calculate the difficulty of the found shares?

SHARE FOUND - (GPU 1) (DIFF: 6.130G)
SHARE FOUND - (GPU 1) (DIFF: 4.566G)
SHARE FOUND - (GPU 1) (DIFF: 6.724G)
SHARE FOUND - (GPU 1) (DIFF: 20.246G)
more ....