This comes down to implementation. I can confirm you have the operate mode for the GSATA (Marvel) chip set to AHCI which is correct.
Marvell
88SE9172
The Marvell®
88SE917X and 88SE9182 host controllers are ideal solutions for small
embedded systems needing SATA connectivity or systems with special temperature or
power requirements. These SATA controllers allow a PCIe-based host system to control up
to two 6Gb/s SATA devices via direct connection. Built-in support for SATA port
multipliers with FIS based switching ensures system scalability with maximum performance.
Older systems:
South bridge, storage controller and MCH is now incorporated in the Z68 northbridge.
Your Z68 board:
The 88SE9172 is PICe 2.0 and only has 1 PCI lane, so if you connect 2 SATA6 disks they will only operate at SATA3 speeds because they are sharing 1 lane.
These chips are connected to the Z68 and in the BIOS are emulated as IDE. This is how support was implemented in this case.