I believe it translates as 5 General Service Battalion which no longer exists. It would have been folded back into 5 Service Battalion a couple of years ago, whose flashes read 5 Bn S I believe. 1,2, and 5 GS Battalions were formed when the Service Battalions were split into GS and CS, and that move has since been reversed.
As a side note, the current Service Battalions are the equivalent of the Tech Services Branches of the ASGs and are not part of the Brigades. Keeping that in mind, anyone want to take a guess as to what organic 2nd Line Log/Maint/Tn a CMBG has?