Sometimes those gaskets make good glue. Typically if you apply some outward force and tap with the rubber mallet eventually it will decide to come off. Trick is to not bend , break, smash, scratch or otherwise deform your cover or engine case.
Maybe try sticking a couple long bolts into a couple cover bolt holes but not into the threads in the case and pinch them together holding the cover and pull out and slightly down or up etc and tap with the mallet.
Maybe try sticking a couple long bolts into a couple cover bolt holes but not into the threads in the case and pinch them together holding the cover and pull out and slightly down or up etc and tap with the mallet.
