After shrinking a BTRFS file system using "btrfs filesystem resize /mnt -20g", this space doesn't show up as free for allocation in GParted. It doesn't seem like GParted can resize the btrfs partition on its own either. Rebooting doesn't help.
The various ways to check freespace on a btrfs volume https://btrfs.wiki.kernel.org/index.php/FAQ#Why_are_there_so_many_ways_to_check_the_amount_of_free_space.3F do show that the total space has shrunk by 20 GB.
How can I reclaim this space to use for another new partition on the hard disk?