Opps, sorry.
US Army OOB and GSR = Ground Surveillance Radar.
Usually a team of 2 - 3 guys with a man-pack radar that sits on a tripod and reaches out 8 - 10 kms for moving vehicles.
With all the obscure acronyms in the OOBs I thought it was just me that didn't know them. Good to know there are others puzzled over some of them too
There is one US vehicle in the OOB with a GSR mounted, but that is all.