Hello.Thomas,
We have debugged Gigabit ethernet card with Intel 82574, But if you want to design 82574 on your board directly, You will have to get firmware from intel and driver source for linux, then porting it to i.MX6 platform.
In addtion, AR8151 is also a Gigabit Ethernet IC with Single Lan PCIe , it is manufactured by Qualcomm. I have discussed the corresponding solution that using PCIe to expend GBE with Qualcomm Engineer. They can provide AR8151 HDK for cusotmer.
I think you can evaluate it .
Regards,
Weidong