I'd include a exception in the license saying that the assets I'm using are covered by your license and not my project's license (GNU General Public License v3). The reason I'm asking is because you probably wouldn't be comfortable with me including paid assets because then an unscrupulous person could get their hands on them by taking them from my project instead of buying them from you. It would be significantly easier to rip the assets from my game because the PNGs are just sitting there in the publicly available source code, and not packed in something like a Unity asset bundle or whatever where it's harder to get the assets. I'm not planning on marketing the game per se, it's just something I'm creating for myself and showing to some of my friends.