Sol,
Seems that in order to calculate geometry on the FGDB data requires that
you be in edit mode for the dataset.
From ArcGIS help referenced above
"Start an edit session
<http://resources.arcgis.com/en/help/main/10.1/01m6/01m60000005p000000.htm>.
1.
You can make calculations without being in an editing session; however,
in that case, there is no way to undo the results."
This is not appear to be completely true for a FGDB. If you are in an
editing session for the FGDB data, Calculate Geometry is available. If you
are not in an edit session, it is grayed out. Shapefiles have Calculate
Geometry available if you are or are not in an edit session for the data.
Thanks, learned something new.