Yes, it is normal for Gabriel's head to spin. He is not possessed.

Call Wolfgang first. Talk to him about "Schattenjager", "Tetelo", "Talisman" (twice), "Tetelo's remains", and "African Homeland" last. After you get off the phone, read the newspaper, then go to St. Louis Cemetery.

Head to the Gedde Tomb, where the door panel will be open already. Press the button and go inside. Someone will have broken the light, so use the flashlight on the darkness. You can check the drawers, but nothing of interest is inside these. Notice the middle one has a veve instead of a name, and it doesn't open. There is an ossuary on the left side of the tomb. Use the gears icon to open a new screen with a puzzle. Five of the skulls will have a gem inside one eye socket. You will need to press these in the right order. Luckily, they left a picture right below this with the correct pattern. That is Top, Bottom, Left, Right, Center. The drawer with the veve will now unlock. Go ahead and open it to find Mosely inside. Gabriel will be hit over the head and lose consciousness for a time.

When you come to, check the drawer again. Mosely will be gone but his wallet was left behind. Take this, then examine it from your inventory to get the Americans Repressed card. That's it for here, so hit the big red button on the right to exit the tomb, then go back to St. George's. You can try calling Wolfgang back, but you won't be able to get him on the phone. Go ahead and call "C" The World Travel Agency, and ask for "Rittersberg, Germany", and then choose to "Charge it" to use Mosely's credit card. Tell Grace you are going to Germany, then exit to the World Map. The Airport will be accessible from the Greater New Orleans map.

You'll arrive at Schloss Ritter, but Wolfgang has already left. First, go through the door on the right to enter the chapel. Look at the three panels on each side, then exit back to the main hall. Close to the chapel doors will be a set of weapons on the wall. Use the gears icon on these to open a new screen. Take the dagger and exit that screen. Take the salt shaker from the table where Gerde is sitting, then go upstairs to the bedroom.

Next to the door is a scroll in a case. Take this, and the scissors. There is a large door across the room which won't open. Read the poem above this door, then head downstairs. Talk to Gerde and ask about "Wolfgang", "Where he went", and "library". Now go back to the main conversation menu and choose "Chapel Panels", "Initiation Ceremony", and then "Portal Poem".

We are now ready to perform that ceremony. Now go back upstairs to the bedroom and open the window. Use the gears icon on the puddle of water. Next, use the scissors on Gabriel. Take the chamber pot and head back down to the chapel.

Once inside, place the chamber pot on the altar, then use the salt shaker on the pot. Next, use the dagger on Gabriel. Kneel at the altar (gears icon), then use the scroll on Gabriel to complete the ceremony. A cutscene will follow, and the day will end.