...

gradients = zeros(length(taus), 1);
for i = 1:length(taus)
  gradients(i) = expdecaygradient(time, voltage, taus(i));
end

subplot(2, 1, 2)
plot(taus, gradients);
xlabel('tau [ms]')
ylabel('gradient [mV^2/ms]')
savefigpdf(gcf, 'expdecaygradientplot.pdf', 10, 10);