How to remove transparent background in GIMP?
The grey/white checkered background is the indicator that you are 'seeing through' to no background - ie that area is transparent.
If you add a layer behind, then fill with a colour, you will see how well your edges have been blended into the background.
At this point, your layer is ready to add the image of your choice behind, with the subject standing in front of it.
If, however, the image still contains the checker, then someone at some point has probably saved a screenshot of the image onscreen, rather than saved the resulting png.
Your file, as posted, is the latter case. That checker is now part of the image itself & no longer an indication of transparency.
This is your image very quickly cleaned up in Photoshop [I don't have Gimp] to isolate the subject from the background. In here it will appear as white but if you re-import it to any graphics package it will be transparent.. & you will again see through to the checkered "not background".
Just for fun, here's your guy on a football pitch - you can see my quick mask left some bad fringing around him; I would normally fix that ;)
