Extending a filesystem partition, from within the same operating system, ismay not be advisable, as its processes / services might still be writing into them. Also, gparted shows certain partitions as locked ( i.e. mounted ) and may not be able to un-mount them.
A Live CD comes in handy, in such situations.
Just use the Live CD from this Open Source Project, on Hosts / Guests ( this Live CD has the gparted program built in, in its Applications Menu, along with many other goodies like File Manager, Disk Mounter, Installer, Internet Browser etc.; also read the instructions on the site ) :
https://sourceforge.net/projects/live-cd-with-zerofree-utility/
P.S. A 32-Bit Version ( i686 architecture ) of this Live CD is available at: https://sourceforge.net/projects/live-cd-with-zerofree-32-bit/
For very OLD Architectures, Please try this Live CD : https://sourceforge.net/projects/live-cd-with-zerofree-x86/
Hope this helps all.