Hard drives keep RAID meta-data on them even after a RAID set is broken. So any drive that was RAID has RAID meta-data. You have to remove the RAID meta-data.
Gparted did not used to work with RAID, but very newest has many updates. But since not a full RAID configuration it may not see the meta-data?
Please run the commands I posted in #28, that will remove the RAID meta-data.