Oh hey! Gouache is tricky stuff to use, you're already better with it than I am. Good work, there!

As for ways to improve, you might want to consider making some of your colors contrast more? For example, since you've got a tan door and a white sleeve next to each other, making the part of the door closest to the sleeve a bit darker so both stand out from each other more could help. You're already incorporating background images so your character isn't floating in a white void, so maybe your next step could be to make the BG more of a character of its own? It doesn't have to dominate the picture, but it could tell a smallish story of its own even if you removed the character. Here you've got the "Stop" floor mat inside the room with bushes and blue sky outside, which is a great place to start: does the person living here hate going outside? Is the warning more for guests, for the house-dweller(s), or just a joke? Putting something simple on the wall on the right side can use up some of the negative space without detracting from the main character, especially since he's wearing red and already draws the eye with his bright colors.

I'm still working on better understanding backgrounds myself, so take this advice with a grain of salt.