Global Cache Blocks Served - the number of blocks sent to the remote instances over the hardware interconnect. This statistic is a sum of another two statistics:
Global Cache blocks served = gc current blocks served + gc cr blocks served
gc current blocks served - is the Global Cache statistic. It is the number of current blocks sent to the remote instance over the hardware interconnect. This statistics derives from v$sysstat.VALUE 'gc current blocks served'
gc cr blocks served - is the Global Cache statistic. It is the number of consistent read blocks sent to the remote instance over the hardware interconnect. This statistics derives from v$sysstat.VALUE 'gc cr blocks served'
See also Current Block Gets vs Consistent Read (cr) Block Gets for explanation of the difference between Current and CR blocks.