Done, works, but the process was a little bit more complicate.
I’m describing so that it might be of help for someone else
I’m working on High Sierra, and the raid drive is g-tech thunderbolt 2 (only T plugs and usb on the back)
g-raid config does not see the hd in the format out of the box
When new/empty I reformatted to the new file system APFS, now the G-RAID CONFIGURATOR utility sees it (after a little while)
4 then I selected raid 1, executed the utility, and
after that in MAC OS Disk utility I reformatted it as asked (for some strange reason I guess one of the partitions is still in Mac Os journaled) and the disk appears as raid with 6Tera. I hope it works as Raid1 now
the process failed a few times (the Configurator gave an error message after starting the RAID1 conversion process, so I reformatted back in MAC OS journaled, and then back to APFS and started the process again)