can you just replace the front break pads on a 2017 GLS 550 or are the rotors

Typically on these vehicles, the brake pads and brake rotors should be replaced at the same time.

Mercedes Benz !

Theoretically you can replace only the pads but the correct way to do it is to replace the rotors as well, or at least get them machined/"turned". If you only replace the pads there is a good chance the brakes will make noise even if they are functioning properly. You also probably have brake pad sensors which will need to be replaced as well. Your brake job will not be inexpensive, and it's important to use top quality parts on a vehicle like yours. Part of owning a mercedes