I dual boot Parrot and Ubuntu Mate. I use gparted to shrink partitions and make swap partitions. You must use the mounted partition to shrink the unmounted partition or use gparted on it's own media. (cd,dvd, usb).
Is this a new single OS installation?
Are you trying to make a dual boot by adding Parrot?
Have you finished your Parrot OS installation but want to increase or add swap?
While the basics are all the same, the steps will be different. More information would help people help you.