You aren't try to run Gparted to resize Solus partition from within Solus itself, are you?
It's not possible to resize, move, delete mounted partitions. Gparted must be started from another installed Linux or from live media - to operate on unmounted partitions.
The simplest way would be to download live Gprated ISO, burn it to USB key, then boot from LiveUSB. After that, you'll be able to do everything you want with the hard disk.
Edit: Or simply boot from live Solus media. It has Gparted as well, so you'll be able to use Gparted from within live Solus session.