I would like to increase my Linux Mint partition (/dev/sda3) with Gparted through a live Ubuntu session. However, Gparted doesn't increase my Linux Mint partition even though it's not mounted and has swap off. I can increase my Windows partition (/dev/sda1) which is formatted as ntfs through Gparted. Is it possible to resize the Linux Mint partition through Gparted or will I need to use terminal commands?
I'm guessing you'd be increasing the partition using some "empty" space on your partition? If you are resizing your windows partition, and it's right beside a linux one (/ or /home), you'd be in essence "moving" the start of that linux partition, could cause some headaches. Not unfixable headaches, but headaches nonetheless.
Following is a quote from the GParted Manual that explains why partition sda3 cannot be increased in size:
To grow or move a partition, unallocated space must be available adjacent to the partition.
If you are growing a logical partition, then the unallocated space must be within the extended partition.
If you are growing a primary partition, then the unallocated space must not be within the extended partition.
You can move unallocated space to be inside or outside of the extended partition by resizing the extended partition boundaries.
For more details see GParted Manual - Resizing a Partition.
Note that if you move the start of a partition then as mentioned in the other answer you may need to restore the ability to boot. This is also described in the GParted Manual - Moving a Partition.