Here is how you can change the exponential damping:
Clear[dosc, gR];
dosc[expDamping_] :=
20 + (1*((Exp[-(expDamping*t)])*((4*Cos[3/10*(t - 0)]) + (4*
Sin[3/10*(t - 0)]))));
gR[t_, r_, expDamping_] := t*Cos[r] - (dosc[expDamping])*Sin[r];
hR[t_, r_, expDamping_] := t*Sin[r] + (dosc[expDamping])*Cos[r];
Manipulate[
ParametricPlot[{gR[t, r, expDamping], hR[t, r, expDamping]}, {t, 0,
75}, PlotRange -> {{-10, 100}, {-100, 50}}], {r,
0, -1.5}, {{expDamping, 0.02}, 0, .1}]