Check similar age / spec cars on sites such as Auto Trader, and get prices from some of the car buying websites such as WBAC, Motorway, Wizzle. Once you’ve done that, it might also be worth checking out what your local VW dealer would give you for it. Most dealers are short of good quality used stock due to new car supply issues, low supply and high demand for new-ish used cars, so you might be pleasantly surprised.
What are you looking to replace your GTI with?