From the pictures its feet look really red, which is not usually a sign of niacin deficiency, so it's very possible they're irritated. The trouble to walk is definitely a sign of niacin deficiency though so keep up with that.
The watery eyes are a bad sign. The black you are seeing around the eyes looks like discharge dirtying the down there.
Can you think of anything it could be getting into that could be irritating its skin and eyes? I saw the mention of hogweed, and that might be it. Can you bring it in and let it bathe in fresh clean water to rinse it off? You might even want to use some Dawn dish soap to make sure you get anything that might be on it off. And it wouldn't be a bad idea to irrigate its eyes with a saline solution to get those cleaned out either.
Also, I read back and it looks like you're feeding a regular poultry ration to these guys. Is that like an All Flock type feed or a layer type feed? How much protein is in it? Goslings need to be on feed for baby poultry, such as a chick starter or a waterfowl starter if you can find one, or an All Flock/Flock Raiser type feed. A regular poultry ration won't have what they need nutritionally.
