Everything here is from the references shown below, with a few comments in an alternate color.
During APU start the AHRS 1-2 DG Mode (caution) CAS Message may display momentarily.
Gulfstream GVII-G500 Airplane Flight Manual, Revision 4, August 29, 2019
Gulfstream GVII-G500 Production Aircraft Systems, Revision 3, July 15, 2019
Copyright 2019. Code 7700 LLC. All Rights Reserved.