We've all made that mistake
I blame brewing classic styles for not discussing malting differences and malt substitutions. There should be a big red warning in that book on any of the recipes that include munich, chocolate or roast barley.
You might not get much more attenuation using gambrinus munich at that high a percentage but it might still be drinkable at 1.020. Keg it up and see.
and I agree with not wanting to throw good at bad, if its only a 5gal batch just brew again. You could try blending a few litres and see what its like but don't mess up a second batch trying to fix the first. I would definitely try souring at least a few litres. A chewy dunkel might bug out to a decent flanders brown.
I've liked the dunkels I've made with 50% gambrinus light munich, 50% pilsner or 70%pilsner, 30% gambrinus dark munich better than the one time I did 100% weyermann light munich...maybe it was the ferment, but i just found the gambrinus stuff maltier.