Depending on which version of the G-RAID you have the configuration is different. If you have one with TB and USB on it then you can use the configurator to change it to RAID1. The process then is to just erase it as one large Volume. You don’t want to partition RAID volumes.
Our recommendation if you wish to use a drive for Time Machine then it is best to use an entirely different drive. G-RAID for storage/working drive and another for backup.