Exhaust in water - gasket?

[Preben]

I have a MB E300 TDT 1998 straight 6 that today lost it's power. Got EPC error with yellow text (can drive to repair shop). I thought the issue was related to another error I have had. I drove slowly home. Just 3-4 miles or so.

I saw nothing especially, and neither the water temp or anything else stood out to me.

Came home and started to hear some "bump bump" sounds. I parked, shut off the engine, and found that the sound came from the water expansion tank under the hood. I immediately went back into the car to see what the water temp was. It said about 96 celsius.

Found some cloths, and opened up the tank cap very slowly until water came out.

Surely, after the engine had cooled, I now see exhaust in the water.

So probably the main gasket is bad?

How do I investigate this further?

Take off the inlet and the cylinder head?

Is is most probably only the gasket, or may it be another big issue?

Please advice. Thank you :-)