As slings to sub-adult I kept mine moist high humidity just like any other tarantulas. Once the tarantulas reaches 4" inches that is when I either keep the in moist substrate or not depending on the species that requires humidity at all times. Grammostola grossa does very well dry substrate, however like any other tarantulas you need a water bowl for your tarantula wether is kept humid or not.
Grammostola grossa is a beautiful tarantula that is very hardy and gets large. You will enjoy this species. Very easy to take care of.