Mercedes GLS transmission fault

That sounds rough. A converter failure on a newer GLS with that mileage is surprising. The grinding noise isn't a good sign either. Have you checked the transmission fluid level? Low fluid can cause slipping and rough shifts. Also, is the grinding constant, or just during specific gear changes? A simple fluid top-up might help temporarily, but a stored error code usually points to something more serious.

I haven't checked the fluid level myself, honestly. I'm not really sure where to start with that on this model. The grinding seems to be worse when it's shifting from 2nd to 3rd, or sometimes when slowing down. Is checking the fluid something I could easily mess up?

Okay, if the grinding is mostly during 2nd to 3rd shift, that definitely points towards internal gearbox issues, possibly the lock-up clutch. Checking the fluid isn't terribly difficult, but on a modern automatic, it can be a bit more involved than just pulling a dipstick. If you're not comfortable, I'd strongly suggest taking it to a trusted workshop that specializes in Mercedes or automatic transmissions. They can properly diagnose the converter fault and advise on the best course of action. Continuing to drive it with those symptoms could cause even more damage.

Thanks for the advice! I appreciate you steering me in the right direction. I took it to the shop, and you were spot on. It was indeed the torque converter and the lock-up clutch was defective. The whole thing ended up costing me 2120€ to get fixed. Ouch!