Gain Vs Loss: The number of new likes gained divided by the number of likes lost.
Gain on [Date]: The maximum number of fans gained on a single day during the time period.
Loss on [Date]: The maximum number of fans lost on a single day during the time period.
For more information on Facebook metrics, please refer to the following FB developer resource: https://developers.facebook.com/docs/graph-api/reference/v20.0/insights