You could dicker around with selections, masks, blurs but really all you need is the eraser tool.
On the label layer, it has a yellow dotted line demarking the layer boundary.
screenshot. https://i.imgur.com/T2jJ1jq.jpg
Choose the eraser tool (1) A fuzzy brush (2) suitable size (3) click and drag to draw in straight lines (4), most of the brush outside the layer boundary. Drawing in straight lines https://docs.gimp.org/2.10/en/gimp-tutor...lines.html
Erase round the four sides.
On the label layer, it has a yellow dotted line demarking the layer boundary.
screenshot. https://i.imgur.com/T2jJ1jq.jpg
Choose the eraser tool (1) A fuzzy brush (2) suitable size (3) click and drag to draw in straight lines (4), most of the brush outside the layer boundary. Drawing in straight lines https://docs.gimp.org/2.10/en/gimp-tutor...lines.html
Erase round the four sides.