After resize virtual disk in VirtualBox and resize partition table using gparted in guest system, do the following to let the guest system to recognize the new disk size.
Make OS see the entire partition using pvresize:
$ sudo pvresize /dev/sda1
Physical volume "/dev/sda1" changed
1 physical volume(s) resized / 0 physical volume(s) not resized
Resize the logical volume using lvresize:
$ sudo lvresize -l +100%FREE /dev/mapper/ubuntu--vg-root
Size of logical volume ubuntu-vg/root changed from 29.03 GiB (7432 extents) to <48.85 GiB (12505 extents).
Logical volume ubuntu-vg/root successfully resized.
Resize the filesystem:
$ sudo resize2fs /dev/mapper/ubuntu--vg-root
$ df -h
/dev/mapper/ubuntu--vg-root 48G 21G 26G 45% /
Reference
https://dev.to/schollii/expand-disk-in-virtualbox-ubuntu-18-guest-24eo