To do things straightforward, on the top bar of the desk, click on:
System, then Welcome then a window open, wait for the animation finish..., then click the button Software, then click the fourth icon on top of the window, (Graphic), scroll down to Gimp and click the button Install, then....
Here it starts to be weird...
You NEED to click on the Bulk Queue, (if not you can wait for years as nothing will happen...)
Bulk Queue is in the red circle on the screenshot, then a new page open search the button Apply changes click on it et it will finally start to download ask your password and install...
Yep the last 2 parts are weird... as we expect when we click on the install button it should... install... but it does not...