Will the roof spoiler(D11) come standard with the GLS 450 ? It says price TBD on the order guide but when I looked up pictures online, it’s pretty much on every 2020 gls availble at dealerships nationwide.

Roof spoiler is not standard. Also it's not on every 2020 GLS available. You must be mistaking the top portion of the tailgate for the spoiler.

Correct, the small overhang on the tailgate is not the spoiler. I had the same wrong impression for a while.

Does anyone know the part number for this? I'm trying to find it online, but unable to locate it.

I see that spoiler as an accessory on MB’s website for the car. Maybe call the parts department at your local dealership and ask them? We have it on ours, and to be honest, I see a future repair someday much like the little trunk lip spoiler on my w212 that has needed to be reattached twice now for failing adhesive...

If the dealer puts it on there is a chance it might need to be reattached. This of course depends on how long you plan on keeping it. Personally I don't really see the purpose of a spoiler on a GLS, especially a 450. It's not an AMG and really wont change performance (aero).

Like most spoilers it's for looks.

I got mine because the rear window on my previous GL350 got covered with dust very quickly, so I'm hoping the spoiler will change the airflow enough over the tailgate to improve this. I only have about 800 miles on the new GLS but so far, it seems to be helping.

I got the root spoiler option completely out of the looks alone. I highly doubt that this is too functional in any meaningful ways in everyday drive.
I really like the look of it.