It feels so good to upvote the two answers ahead of mine. I am only commenting because I feel strongly that while I grok their answers, I feel a plain-English interpretation of the solution would be better appreciated by some. Here goes:
While the observation of the first answer is correct, and yes, your terminal command was in error for Ubuntu, your best bet is to use the graphical software installation utility to install gparted. The following simplified instructions will permit anyone, with even the most minimal understanding of any operating system at all, to install GPARTED in less than one minute.
- Press your Windows key.
- Type the keyboard characters SO When the Ubuntu Software Center icon is displayed, click on that, or simply press your ENTER key.
- When the Software Center has loaded, type the keyboard characters, GPA
- When the Gparted Partition Editor option appears, click that. It should turn orange.
- Press the INSTALL button.
- Wait 20 seconds while it installs.
There is nothing wrong with becoming proficient with terminal commands. It is especially useful if your goal is to compose scripts for complex installation problem solving. However to simply install one program such as GPARTED, the software center is actually faster.