Smash is free so it doesn't matter if you are using it as a gravy thickener, it wouldn't have to be synned.....well, according to my group leader anyway.
I have done this with my gravy but have to admit that it only 'does the job' it certainly doesn't have me jumping for gravy based joy.
Something that does work for me is making gravy with an oxo cube and then putting some of my superfree veg into the gravy, wizzing is up with a hand blender and using that as a thicker gravy. Much tastier, if you can stand the tiny bits. It works really well to do that if you are slow cooking anything, as I find the slow cooker leaves everything really watery so wizzing some of the vegies really helps make a proper stewey gravy.