i doubt it has a GTI rear axle if it has drums on it. otherwise someone went to the hassle of swapping over the rear stub axles to run the drums again!
only difference i cant think off between GTI rear axle & std rear axle is the incorporated rear anti roll bar
anyhow, to answer your question the easiest way is to get a complete rear axle including disc's & calipers, and replace your existing one. cheapest/easiest option
and its your front brakes you should really be upgrading, theyll be piddly 239mm discs