That's not a top cap. That's a conical spacer. Its OK if it spins round. As long as there is no back and forth play between the fork steerer and headset everything is OK.

The top cap is above the stem. If you lossen the stem bolts then tighten the top cap bolt. Then re-tighten the stem bolts. The gap will be eliminated. Its still normal for the conical spacer to spin abit.