Most people with a functional gallbladder don't have any problem breaking down "fat" - they may have issues with certain types of fats (like I can't handle olive or grapeseed, but I do fine with sunflower or walnut), but fat across the board shouldn't be an issue.
Fat is very difficult to eat alone - it's pretty much always combined with something else in nature. Usually protein (like in meat or nuts), occasionally cellulose (like in avocados or olives). If you're eating a balanced diet with vegetable matter in conjunction with your protein/fat sources, you shouldn't have any difficulty. I don't think grain is necessary for fat metabolism (although I do think fat makes grain metabolism work better).