I don't think the GPT partition table is your problem (the original warning from fdisk is normal in this case), I would ignore that. You aren't trying to make the drive bootable, and parted read it just fine.

I honestly don't feel comfortable trying to advise you further (bad advice is worse than no advice imo) - it *may* be as simple as making sure you have the mdadm package (and possibly lvm2) installed and rebooting and seeing if it automatically detects the raid volume and going from there - I hope I've at least given you some keywords to search for