In your first run of the level, you'll never, ever get a gold!
Getting a silver meaning you're good to go. Just run through the level a second time doing only the necessary to pass. You'll not even get a bronze medal (well, I got loser medals each time rushing through the level a second time), but when you return to the world menu, it'll show a gold. This is because the game captures that you've already done enough to get a gold medal the first time you play it. The only thing preventing the gold on your first playthrough is the second costume for each level.