DriverGestaltOn and DriverGestaltOff let driver code indicate to other software that it does or does not support driver gestalt.
OSErr DriverGestaltOn(DriverRefNum refNum);
OSErr DriverGestaltOff(DriverRefNum refNum);
DriverGestaltOn and DriverGestaltOff set and clear bit 2 in the device control entry (DCE) flags word.