The decorative squares are inspired by a Photoshop speedart video, which i tried to replicate in Gimp, but wasnt very successful.
Photoshop has vector shapes and doesnt clip layers to the background which is a huge advantage when you want to rotate objects that are bigger than the canvas (Gimp cuts everything of).
Also clipping mask are really handy.
Then it seemed arakne's shape creator doesnt work in Gimp 2.10.
I also got some odd edge behaviour from the layermask and scaling or rotating selections doesnt work very well either (never tried that before). So making these diamond shapes was too difficult.
Next i gave it a try in Inkscape which wasnt much fun either, but i got it working.
The Object Manager is basically a very good addition to the program but needs a lot of improvement to be of any real help and applying gradients is absolutely always a mess.
I tweaked the final result in Gimp, because it looked a bit dull (contrast, brightness via Curves and a HighPass Filter). Another thing that is hard to change in Inkscape.