For example, here is the diffuse layer:
ac8.jpg
Then the ambient occlusion on top in 'multiply' mode.
ac9.jpg
And then the shadows on top of that also in 'multiply' mode.
ad1.jpg
There are all 8 bit and really can turn into a mess if that's all I use.
I think Gimp 2.9 is great, but until there is non-destructive editing, PS is going to always be better.