Currently the ticks and grid line spacing are associated and so the only way to change grid spacing is to change the tick spacing. Here are some examples:
figure;
plot(1:100);
set(gca,'xtick',[0:13:100]);
set(gca,'ytick',linspace(0,100,13));
% The following code changes the minor grid
% spacing by adjusting the tick spacing:
figure
plot(1:100);
grid on
grid minor
set(gca,'xtick',[0:50:100])
set(gca,'ytick',[0:50:100])