If I were making a gallon of Gritty mix, how much garden lime should I be adding to the batch?
Use 2 tsp - 1 tbsp gypsum/gallon as your Ca source for the gritty mix if you're not using Foliage-Pro or a fertilizer that contains Ca. If you're not using FP, you'll need to add a (very) little Epsom salts to your soluble fertilizer every time you fertilize.
Why don't we use Garden Lime in this mix (since it has CA and MG, the Epsoma brand)? Is it pH related?
My fert doesn't contain CA or MG.
Yes - pH related. Bark/peat based soils have a much lower starting pH and need the lime. The gritty mix has a higher starting pH, and gypsum (CaSO4) doesn't impact pH; nor does the MgSO4 (Epsom salts) you'll use along with it.
Thanks for explaining! It makes sense now. If I use Foilage Pro, can I skip the gypsum and epsom salt?
Yes - many of us do just that.