As always:

D O   A   BACKUP. At least of the sensitive data on the partition, you want to shrink. Neither I, nor geparted can be held responsible if you screw up.

Now to the procedure using gparted:

The picture is a little messy but don't get scared (you'll be moving with your partition to your auntie and uncle in Bel Air. Wait wat? XD). Just stick with this for a moment and follow the instructions below. Start looking at the image in the top right corner. Then follow the numbers in ascending order.

Here is a text version of the steps numbered as in the picture:

• Install gparted / sudo apt-get install gparted


• Start gparted
  
  
  
  
  1. Select the disk in the upper right dropbox
  
  
  2. Select the partition in the main list in the middle
     
     
     
     
     • If mounted: Unmount that partition using the contex menu (right click)
     
     
  
  
  3. Click the resize button
  
  
  4. A dialog will pop up
  
  
  5. Resize
  
  
  6. Click "Apply" after double checking your changes. That's it.
  
  

This will not work with all partitions. It can be more complicated for Mac partitions or logical volumes.