For the first 3 to 5 years the manual GT3 will trade at a discount to the PDK ones. As the performance edge of the car will fade into oblivion, manuals will eventually have a premium due to the relative rarity and nostalgia, 7 to 10 years horizon.

Personally I think that manual make no sense in this decade and even less on a performance sport car. My 612 manual is worth twice the price of the F1 but that's because in today's world F1 tech is obsolete and there are only 200 manual 612 in the world. To think the same will happen when there will be over a thousand manual GT3 plus all the older ones around is far fetched.