Cutting image borders

[konjon]

Completely new to gimp here, so I've been trying to do this for about an hour and it was a complete failure. I understand this is a complex software so the solution might be simple but not intuitive for begginers. I want to remove the sides of the image so the edges of the image are as close as possible to the phone .



I want to remove the red highlighted zones and then export the new image with alpha layer in the back. I know there should be like a million tutorials about this, but I couldn't find any adressing this issue directly. Any help?

 I attached the original image that i want to edit just in case.