Hi,

I installed TrueOS and it partitioned the whole disk as a zfs pool. Then, while installing Archlinux I used parted to create a new gpt table with fat32, ext4 and btrfs partitions. Archlinux worked well. But Gparted reported, that the whole disk still was formatted to ZFS. I reproduced the same scenario on a virtual machine - the same result.
It turned out that there remained signatures from ZFS.
Here is what blkid and wipes reported:

root@debian:~# blkid /dev/sda
/dev/sda: TYPE="zfs_member" PTUUID="f80e1ce2-028f-4124-bdef-ce5a61ad4e72" PTTYPE="gpt"

root@debian:~# wipefs /dev/sda
offset type
----------------------------------------------------------------
0x200 gpt [partition table]

0x7f000 zfs_member [filesystem]

I had to manually erase all old signatures with wipefs to make Gparted see my partitions.

I'm attaching a screenshot of the virtual machine where I reproduced the scenario.