I know this is a bit old, but G thula are not picky at all. By all means, go with the best you can give them, but I can confirm that they will do fine multigenerationally on borderline garbage substrate.
Without going into the details, money got really tight, and I ended up raising two generations of G thula on organic top soil from (like $4 or $5 for 40lbs) and just straight-up dried shredded oak leaves. They weren't rotten at all, just freshly collected from the surface that season. The larvae ate the leaf material with no problem, only leaving the steam portions, and went through quite a bit of the top soil as well. The adults came out fine, bred, lived a long time, and their offspring hatched and lived entirely off of that same sub (with a fresh introduction of the same dried shredded oak leaves), eclosed to adulthood the same size and colors as their parents, and similar bred and are currently ovipositing.
Like I said, I'm sure they would do better with higher quality sub, but they do okay on sub-optimal materials.
Somewhat related: the females will lay eggs on a flat barely moist paper towel. #yolo apparently.