I'm not sure at what point you stop calling something a "garden tractor" and start calling it a "compact tractor" instead. There are many compact diesel tractors out there around the same size as many garden tractors - but with more rugged drive-trains.
Deere made a 318 garden tractor with a Peerless gear transmission that was rugged enough. 18 horse with a 50" mower.
Wheelhorse had the D250 with a gear transmission and a 20 horse, four-cylinder water-cooled engine. Came with a 48" or 60" mower.
You could buy a compact Yanmar with a Deere name-tag on it like a 650 or 750 and get a rugged gear-trans.
There are many, many makes out there with a gear trans.