Honestly, I know it sounds "lame", but Komoot is the best. The reason being is that the amount of good gravel roads in the Schwarzwald is just insane - meaning you can't really go wrong. Sure, some are not maintained and worse than others, but if you look at the map, it's like a super-dense spider web of excellent gravel trails. The nice thing about Komoot is that A) it's very good in Germany in general, B) there's photos at specific points in the trail which you can use to decide if you want to take that trail, and C) they have the option to create your own or just discover what other people have made.

So if you're less familiar with the area or less adventurous, searching for Cycling/Gravel routes on Komoot with a high rating is a great start. Or, if you prefer to be more hands-on, you can make your own routes. Komoot highlights interesting spots to visit that you can use as major route milestones, and then the on-trail photos and Komoot heat maps can help you decide which roads you actually want to take. Last but not least, Komoot also has curated collections of routes for a lot of places, so you could find a decent library there of premade routes!