Hi when I purchased my A7 2017 from a reputable dealer, I didn't receive any Audi manuals and don't really know what service intervals is specific to my car. For gearbox oil replacement would this be every 50k miles? Also, fuel filter replacement would this be every 100k?

It depends if the car is set up on long life servicing or not. Audi would be able to tell you. And inform you of all the work it's had in its life by Audi. 

Filter change may depend on engine but I think it's roughly 5 years or 50k. Whichever comes first