This response is based on the information provided.  We suggest you discuss with your contracting and finance team, program manager and/or legal department as appropriate.

The FAR does not address service specific implementation of agency practices, so no, the FAR does not direct GPC use instead of Army Supply System   OCONUS.  Your Brigade (BDE) is creating the rule to go through BDE rather than use the GPC locally.  In an manner of speaking, BDE is following FAR part 8, although that may not be its direct intention.  It's point appears to be that the Army already holds the items in question and therefore should not purchase more.

Your best approach would be to indicate that the Army approach does not function well under your unit's particular circumstances (geographically separated unit).  Indicate with data how it is more expensive and/or more time consuming to go through BDE rather than use the GPC locally.