YouTubes have shown it's anything but that--beautiful rolling hills
and a lot more open than I had originally thought.
Gengenbach is one of the nicest towns to visit in this region, but getting there by train via Offenburg is not at all what you are looking for in terms of scenery. It's a boring train ride across the flat plane created by the Rhine River.
You should first take the Hell Valley Railway line to Donaueschingen which climbs through forest and mountains to Donaueschingen (the source of the Danube River.)
https://commons.wikimedia.org/wiki/Category:H%C3%B6llentalbahn#/media/File:Verlaufskarte_H%C3%B6llentalbahn_2.png
This direct train ride takes you from 928 ft. (Freiburg) to 2267 ft. (Donaueschingen) in 1 hr. 30 minutes.
Route info: https://en.wikipedia.org/wiki/H%C3%B6llentalbahn_(Black_Forest)
From Donaueschingen there are direct trains every 2 hours to Gengenbach (elevation maybe 500 ft. or so.) (There are also additional journeys which require one connection in between Donaueschingen and Gengenbach.) Your train will use the most scenic railway in the Black Forest, the Schwarzwaldbahn (German for "Black Forest Railway,") and the trip takes 1 hr. 15 minutes.
https://upload.wikimedia.org/wikipedia/commons/4/4d/Verlaufskarte_Schwarzwaldbahn_%28Baden%29.png
Route info: https://en.wikipedia.org/wiki/Black_Forest_Railway_(Baden)
If you don't wish to linger in Donaueschingen, you will have a 30-minute transfer period in Donaueschingen. No sweat. Sample schedule for Freiburg > Donaueschingen > Gengenbach:
7:42 - 10:53
9:40 - 12:53
Then you'll have a few hours to enjoy Gengenbach and have a nice meal, after which you take the boring train ride back to Freiburg via Offenburg. Once back, you can say you did the best you could possibly do to tour the Black Forest in a single day.
There never has been a direct train ride between Freiburg and Gengenbach AFAIK, and I've been visiting the Black Forest off and on for over 30 years. I would not expect the 2027 schedule to be any different. When he schedule does come out, I doubt you will find much change if any as far as routes, transfers, and travel times are concerned.
At the station, buy a Baden-Wuerttemberg Ticket (day pass) for two persons (€36) and all your travel is covered. It is valid all day long on any regional trains and/or other means of local transport:
https://www.bwegt.de/tickets/bwtarif-tickets/baden-wuerttemberg-ticket