Constructing a grenade (for a novel I'm writing.)
In short-no your technique will not work.  Reacting limestone with acid does not release enough energy to reach an appropriate temperature to ignite gunpowder and the acid would hinder combustion.

Fear not though, spontaneous ignition may be achieved by mixing glycerol and potassium permanganate which works well and is a classic example.  If you would like to be more out of the ordinary, zinc powder and ammonium nitrate with a minor amount of copper or ferric chloride and water will also create a spontaneous ignition for your gunpowder.