Is it true that when a spring is extended, potential energy is stored in spring and result in gain of mass of the spring?
Yes, spring mass will increase but in very very small amount.
As per Mass energy equivalence the amount of mass gain will be(E=mc2):
for example if spring have store 1 Joule of potential energy when it is extended then the increased mass will be:
m =1/299792458= 3.33564095198152e-9