I've never found the skins on Sun Gold tomatoes particularly tough, but we cook down any surplus tomatoes with have with onion and garlic, and then whizz with a stick blender to make tomato purree for the freezer so the state of the skins never bothers us. I wonder if your Sun Gold skins are tough because of lack of water. You can often find that in very dry summers, tomatoes do grow thicker skins as a form of protection. My Sun Gold skins don't seem to be any thicker than the Gardeners Delight, but mine are all watered copiously on an almost daily basis unless it has been raining.