Lets jump straight into it.

Step 1:

Click on the Global Tools button

Step 2:

Select the appropriate disk name that your virtualBox uses.

Adjust its size to the value you desire.

We will be increasing its size from 20GB to 60GB.

Click apply before going back to Machine Tools.

Step 3:

Create a new virtualBox machine.

Name it gparted and go to the settings.

Load the gparted iso you downloaded.

Load the vdi of the machine you want to increase the hard disk space.

Go the system tab and change the sequence of booting.

untick floppy make sure the optical disk loads first then the Hard Disk follows:

Click okay.

Step 4:

Start the gparted vm.

Click okay.

Click okay if its the language you prefer.

Click okay.

Step 5:

Resize according to your intended hard disk space.

In our case it will be 50gb.

Click resize.

Save by clicking the green tick.

Save details.

We are Done.

Step 6:

Close the vm.

Boot up the vm you have added the hard disk space

To verify

It has worked

References

1. https://www.youtube.com/watch?v=ozJmfSPcfxY

2. https://linuxhint.com/increase-virtualbox-disk-size/