Suspension parts should be replaced in pairs, as age and wear effects them. A 100k spring will be more compressed than a fresh equivalent.
If it is a new GTI/Ed30 spring fitted then the other side is most probably shot.......or as you say a non GTI spring was fitted.
BUT as mentioned it should be done in pairs (suspension struts, springs, brake discs, pads, bushes etc), to make the handling and braking even