There are 3 things that can make you unable to expand the C: drive with GParted. Here’s how to fix them so you can expand the C: drive with GParted.
Firstly, you must rearrange any other partitions so that there is contiguous space immediately after the C: drive partition.
Secondly, you must not be trying to allocate space from an extended partition.
Thirdly, if the you’re unable to expand the C: drive and points 1 & 2 are covered, then you have some errors on your file system that are stopping you from expanding the partition. To fix this, run the following command in the command prompt: chkdsk /F /R
Press Y, then reboot and wait ages. You should then be able to expand you C: drive.