How can I stretch an image like above?
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:
I don't use gimp but from a quick google and youtube search I think the tool you are looking for is the Warp transform tool.
this short video demonstrate how to use this tool (i think it's in Dutch but you can still get the gist)