Gptsync is a utility to maintain an mbr partition table synchronised with a gpt partition table used as reference. the resulting mbr partition table can contain up to 4 partitions; extended partitions are not supported.

an mbr partition table is required to use legacy bootloaders (lilo, grub) on efi-based (extensible firmware interface) machines like the intel-based macs. gptsync is usually used in combination with the refit boot menu on such machines.