Can't resize APFS container
Just for the record. I've managed to increase the size of my APFS volume using GParted and Disk Utility (from macOS Catalina).

Create a GParted USB drive and boot from it
Use GParted to move the APFS partition/container around so that the free space appears directly AFTER the APFS partition.
Boot into Mac Recovery mode and open Disk Utility
Click the drop-down in the top left and enable &quot;Show All Devices&quot;
Select your SSD (top entry)
Click &quot;Partition&quot;
Somewhat unintuitive, but now select the &quot;Free Space&quot; area and click the &quot;-&quot; button below. This will &quot;remove&quot; the free space and enlarge the APFS container to fill it.
Confirm the dialog and the resizing should start.
Done.