According to my manuals, apart from generator power, GCU 1 can be powered by the Battery Bus and GCU 2 can be powered by the DC Standby Bus.
Airplane logic will not allow the BTBs to open if the Transfer Switch is selected to OFF when the aircraft is on External Power. With the application of external power, both BTB's are commanded to close (if not already closed).
If the GCUs are not powered when Ext Power is applied (and, for some strange reason, the BTB's were open prior to power being removed from the aircraft), the BTBs cannot reclose. This situation, however, seems unlikely to me.
Rgds
NSEU