Very common problem !!!!! I had the same thing very frustrating !!!!!!
Good news its very fixable but you cant really do it yourself, its one of two things
when you seclet a gear esp the reverse and first gear there are bushes which separate each gear these do wear out and need replacing you can buy a kit, also the linkage can come out of sinc quite quickly esp when you force the gear and the brackets can be bent these need reajusting, any good Grage that specilaises in Golfs can this .... dont take it to any old garage like I did to start with absolutle nightmare
hope this helps
Greenie