Can the GPC be used to purchase Insignias? If not, are there any other avenues to make the purchase?
The DOD Government Charge Card guide book cites a list at DFARS PGI 208.7006 of items that can be purchased from DoD components responsible for providing the commodity. As you scroll down the list, you can use a MIPR to purchase "badges and Insignias" from DLA. If they are locally available, you can purchase the Generals aide inignias with the GPC.
However, be aware some services (i.e. USMC) make a distinction between identification badges available through the supply system (like a Master-At- Arms badge, military police, or command senior enlisted badges) and Presdential service, Joint Chiefs of Staff (JCS) or OSD badges, which are personal uniform items. The former can be purchased with the GPC, the latter cannot. Check your services uniform regs for further clarification if needed.