Based on your description of your bike, if you want smaller gears, I think you are best off buying a complete new bike. The replacement options for your set up are limited, given the age and set up. Gebhardt may still make replacements, but it is unlikely you can go smaller up front. You might be able to find a larger cassette. I haven't shopped for 6-spd cassettes in awhile.

And trying to put a modern drivetrain on that bike would be very expensive - would need to replace drivetrain and wheels, and maybe widen the rear spacing on the frame.