but it doesn't allow me to resize existing partition.
Yes it does. One reason it will fail to do this is if the partition you are trying to resize is mounted. Right click it and unmount it first. Also if you are trying to resize a partition that is not at the end of your disk then not only the partition you are resizing, but all the higher numbered partitions must be unmounted also. Of course it goes without saying that you can't resize the partition you are running on. And if you have somehow managed to run gparted without it asking for your password first (unlikely but I have known it happen) then it won't do anything at all as it needs root (sudo) privileges. Lastly you have to have some free space to resize into if you are trying to increase the partition size.
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