Based on your stated interests in the Black Forest, your plan to stay in Gengenbach makes good sense to me. Some details on worthwhile places Dave and others have mentioned...
The BF open-air museum (Vogtsbauernhof) is excellent... 25 minutes from G'bach by train to the "Gutach Freilichtmuseum" stop.
Gengenbach is on the scenic Black Forest Railway, an engineering wonder for its time and one of Germany's most scenic train rides. In theory, you could ride this railway on just one train all the way to Konstanz on Lake Constance (at the Swiss border) which would require 3 hours, something you might want to do on a rainy day. However, the KONUS card is valid only to Donaueschingen (the source of the Danube River) which is a trip of roughly 65 minutes; on the way you'll pass through Haslach, Triberg, and Villingen (a larger and very attractive medieval town that Rick Steves apparently never bothered with) so stops along the way make sense.
BF Railway map: https://upload.wikimedia.org/wikipedia/commons/4/4d/Verlaufskarte_Schwarzwaldbahn_%28Baden%29.png
Villingen: https://www.villingen-schwenningen.de/en/tourism-experience/experience-vs/places-of-interest/city-district-villingen/
Haslach, another very pretty old-world town. The Costume Museum might interest you.
https://www.tripadvisor.com/Attraction_Review-g2291703-d3474405-Reviews-Schwarzwaelder_Trachtenmuseum_in_Haslach-Haslach_im_Kinzigtal_Baden_Wurttemberg.html
Looking for a walk? There is a walk you can take - the "Walk of Remembrance" which memorializes the experiences of the forced-labor prisoners during the Nazi regime and takes you out of town to a former work site, now memorial site, called Gedenktstätte Vulkan. (The signage there is largely in German.)
https://www.gedenkstaetten-suedlicher-oberrhein.de/en/gedenkstaetten/haslach/
(We managed to get ourselves a bit muddied in our tennies, so consider the conditions before setting out.)
Flyer (German) with map of the walk, if interested: https://info.haslach.de/_Resources/Persistent/cb04b62af582cab24fff340b061ca33fe34b9d0d/Flyer%20WdE.pdf
Schiltach is very attractive... 35 minutes from G'bach. The train ride to Schiltach from G'bach takes you on a detour off the Black Forest Railway and onto the Kinzig Valley Railway; you could easily spend a day poking around in Schiltach and the other towns on this route:
Kinzig Valley Railway
Alpirsbach brewery: https://de.academic.ru/pictures/dewiki/86/Verlaufskarte_Kinzigtalbahn.png
Wolfach glassblowing: https://www.dorotheenhuette.info/1/
Freudenstadt: https://www.tripadvisor.com/Attractions-g187282-Activities-oa0-Freudenstadt_Baden_Wurttemberg.html
Freiburg makes for a really nice city-outing from Gengenbach. It's 1.25 hours from G'bach by train. Be careful not to use the ICE or other long-distance trains that are on this route with your KONUS card, which is only good on the regional trains.