If you need a 'logarithmic' type of scaling there might be a plugin - don't know of one.
Using standard Gimp 2.10 tools, no easy way that I know of. My thoughts.
Progressive scaling in sections.
Start with this, Canvas is resized to accommodate the extra width. Selection of the outer '2 strips' Throw in some guides as required. screenshots: https://i.imgur.com/EjGRYpi.jpg
Scale the the selection 200 % https://i.imgur.com/tysiWzD.jpg and anchor it
Select the now resized outer strip and scale 150% https://i.imgur.com/l9yIBia.jpg Anchor it
Repeat on the other side. All very tedious.
Using standard Gimp 2.10 tools, no easy way that I know of. My thoughts.
Progressive scaling in sections.
Start with this, Canvas is resized to accommodate the extra width. Selection of the outer '2 strips' Throw in some guides as required. screenshots: https://i.imgur.com/EjGRYpi.jpg
Scale the the selection 200 % https://i.imgur.com/tysiWzD.jpg and anchor it
Select the now resized outer strip and scale 150% https://i.imgur.com/l9yIBia.jpg Anchor it
Repeat on the other side. All very tedious.