Update: This was just a coincidence, it wasn't the gas cap as most of you probably concluded. When I bought the car I had no issues with the tank I had on the date of purchase. I later filled up at a BP station in my neighborhood and it is with that tank when all the observations below were made. I filled up again at an ExxonMobil and bought the gas cap on that day. I had no issues with that tank. After that tank was finished I went back to the same BP station and I had two episodes that were less severe as the CEL went away as soon as I restarted. Since then I haven't been back to that BP station nor have I used BP gas, using Shell gas instead.

I bought a 2015 GL550 with 53k miles recently. After a few days of ownership the engine started to sputter and shake when driving such that I would have to drive slowly for the rest of the way. The first time it happened after about 10 mins of driving. The CEL started flashing as it happened and it was on the rest of the way home (another 10 mins). I made the mistake of continuing to drive while the engine was like that although at about 50 mph. I stopped in my neighborhood and restarted the engine. The CEL went off and it was as if nothing happened. Drove normally the rest of the way.

For the following few days when I was on my way to work, same thing. 10 mins in it would start and I would stop the car and restart it. It would do it again once or twice and I would stop and restart until eventually the engine was normal with the CEL on. After getting to work (another 10 mins with CEL), I would go for a short ride (about 5-10 mins) and stop to restart a few times until eventually the CEL went off. Everything will be normal until the following morning. Have to say I was prepared for the dreaded cylinder 5 misfire issue and a loss of $10s of thousands of dollars. Oddly mpg was better than my GL450 (2013), I am averaging around 19-20mpg, the GL450 averaged 18.2mpg (lifetime).

Anyway, long story short, I bought a new gas cap for $30. I felt if the CEL goes off then everything else with the engine has to be fine. It either had to be the gas I was using or the gas cap. Fortunately it was the gas cap. These machines are super sensitive I guess such that a 6 year old gas cap is no longer good enough. The symptoms were scary though and one can imagine how much Mercedes would have charged for diagnosis.

Hope this helps anyone else who might have the same issue. Alternatively, just get new gas cap for your machines so you don't have to go through this.