Hello, this is a common way to boot non GPT-aware OS on GPT systems. For this protective MBR is replaced by a so-called hybrid mbr where first partition entry is a protective entry and the rest contains legal partitions. So here is the syntax:
gptsync DISK [PARTITIONSPEC1] [PARTITIONSPEC2] [PARTITIONSPEC3] PARTITIONSPEC says which partitions have to be shown to legacy OS. It always begins with the partition number. Then optionally goes one of the following:
nothing. Autodetect partition type, not active partition "+". Like preceding but make partition active "-TYPE". Use TYPE type as MBR partition type "+TYPE". Like preceding but make partition active
gptsync.diff