What is this icon above the gym I just completed a raid at?

Solution 1:

As mentioned above, this is just the victory icon that appears after you've defeated a raid boss. That icon won't disappear until the raid boss leaves.

While a raid boss occupies a gym (regardless of whether or not you've defeated it), the Pokémon defending the gym are not able to be battled. Due to this, it's not possible to remove opposing teams' Pokémon or put in your own until the raid boss is gone.

From Niantic's support page about Raids:

Once you have defeated the Boss, you won’t be able to interact with the Gym until the raid has completed. After the Raid Battle has completed, the Gym is restored to its state before the raid.