If your version of gritty mix doesn't include pumice, but is the common granite-Turface-pine bark version, you should find the mix to already be rather acidic. I would suggest you start with some pH testing of your mix.
My last two bags of Turface, the water going in was in the mid 8's, and the runoff - after a few hours' soak - was down around 4.

You can always use gypsum, to avoid increasing the pH as you provide calcium.

Additionally, should you in fact elect to acidify things, two techniques come to mind: First, soil sulfur can be added to the medium with good results. Slow-acting but as a consequence, rather long-lasting. The other idea-I use both- is to use an "acid special" type water-soluble fertilizer, at a very dilute ratio, each and every time you water. Such fertilizers have for their nitrogen source either ammonium or urea, or both. Our tap water here in this city comes in at a pH of right around 8.8! Needless to say, for acid lovers, the more one waters, the worse the plants look. Since incorporation this technique, we've managed to avoid the chlorosis problems which can result from a too-high soil pH.

+oM