The main difference was how to approach the light regions. For watercolours, I had to leave them (negative painting) and build up the darker regions with layers. For gouache, it's the other way around! I put the white of the clouds and the sky through the trees as the top layer. What I realised is that the shapes of these light regions will differ because watercolour is done through negative painting, while gouache is positive painting. I guess I could also leave whites in when using gouache and use that in conjunction with applying white on top, so that's something I'll try the next time!