You can use the Warp Transform tool in GIMP
It’s located here on the toolbar shown below, or you can press W to select it. Then just paint with it to move and warp the pixels. You can change the size of the tool using the [ and ] keys. When you have finished hit Return / Enter to commit the warp.
It works best if you don’t go too crazy with the tool, otherwise you will get a lot of blurring.
Example before and after: