The checker pattern denotes transparency. Just exists inside Gimp. Sometime in the past you increased the canvas size or cropped-with-growing or ...several different ways. Exporting as a png image retains transparency, (aka alpha channel) If you export in some formats there is no transparency and the border uses the background colour, typically white. Happens with a jpeg export.
To get rid of the border use Image -> Zealous crop.
If that does not work (sometimes does not) then one way is Layer -> Transparency -> Alpha to selection then Edit -> Copy and Edit -> Paste as New image. Export the new image.
If you have a steady hand, use the crop tool with just "Delete cropped pixels" enabled. and crop out a smaller image.
To get rid of the border use Image -> Zealous crop.
If that does not work (sometimes does not) then one way is Layer -> Transparency -> Alpha to selection then Edit -> Copy and Edit -> Paste as New image. Export the new image.
If you have a steady hand, use the crop tool with just "Delete cropped pixels" enabled. and crop out a smaller image.