In thinking ahead for next year's attempt at a GDevelop game, I luckily discovered
opengameart.org which might fill the hole for visual assets.
Between this and FreeSound, it's actually kind of crazy how many free resources there are to work with. Not being an artist myself, it's nice not to have to worry about having to hand-craft everything, or spend money on assets when I'm not planning on selling or making money with what I make.