WARNING! Backup your data before proceeding!
For me this process didn’t destroy the data. I completely expected it to, but it didn’t. Make sure you backup in case you’re not as fortunate.
Move into Single Users Mode
sudo init 1
Unmount partition in question
sudo umount /mnt/disk1
Launch PartEd pointing at the disk in question
sudo parted /dev/sdb
Make the Partition Table GPT
Accept the warning and quit PartEd
Warning: The existing disk label on /dev/sdb will be destroyed and all data on this disk will be lost. Do you want to continue?
Check the disk (resize2fs required this before it would resize the partition)
sudo e2fsck -f /dev/sdb1
Resize the partition to fill the total available disk space
sudo resize2fs /dev/sdb1
Mount the disk again
sudo mount /mnt/disk1