My question is the following.
Hard disk is fully allocated... What's the best way to shrink one partition and grow another in Gparted?
stackoverflow.com, unix.stackexchange.com, or superuser.com?
It seems most appropriate for Super User.
It's not specific to Unix (from the few seconds I spent reading Gparted's "about" page on their site), so posting there doesn't seem proper. If it's usually used on Unix environments, you are using Unix, and you have reason to believe that something related to your OS is relevant to the answer, then it might belong on Unix, but that would be a bit of a stretch, at least from what I understand of the question.
It's not a programming question, but a question of proper usage of an existing program, so Stack Overflow would also be an impropor place to ask.
Since it is about proper usage of an existing tool, that leaves Super User as the proper place to ask.
Keep in mind that, as is, the question is not of high quality and will potentially be closed if posted. Hopefully you just summarized it for the sake of the meta post. If not, you should elaborate on your situation. You need to explain what you've done so far, any research that you may have done into the issue, and add any details about your current system/configuration that may be relevant. (I couldn't tell you what that might be, given no knowledge of the tool.)