As gparted doesn’t seem to support ext4 you should resize your partition manually.
It sounds difficult but is trivial.
First delete the partition with fdisk, then recreate it again in the desired size, resize the filesystem, done.

In some more detail, assuming the partition is /dev/sdb3

umount /dev/sdb3

sudo fdisk /dev/sdb
(remove sdb3)
(recreate sdb3 with the same start position as the previous one)

resize2fs /dev/sdb3 (when no size given the whole partition is used)

mount /dev/sdb3

That’s it.