It seems that snapcraft when building gadget snaps has a check at the very end that there is a gadget.yaml in the root of the project directory, but snapcraft remote-build does not copy this file to the generated launchpad project, and thus it does not properly build gadget snaps. I took the exact same project and uploaded to to a lp git project and created a snap recipe and it just worked fine, presumably because the gadget.yaml file was there.