Granny Smith has the highest level of malic acid of any apple.... and the naturally occurring l-malic acid form, not the junky synthetic d-malic usually sold as a supplement.
Also, don't discount any bitter herb's ability (including milk thistle) indirectly to dissolve gallstones. For one, they trigger the tongue's bitter receptor to signal the liver to create fresh bile. Especially milk thistle has the ability to heal the liver by creating new liver cells.
Though a better way to dissolve gallstones, especially heavily calcified stones like kidney stones, EDTA, IP6 and magnesium. Another good way to dissolve Gallstones is to supplement with lecithin, malic acid, taurine and glycine.