Crunchy gear changes are often the sign of the synchromesh gears getting tired, and if so, it will gradually get worse and worse.
It seems to happen more with 2nd and 3rd, but all gears can be affected.
You can sometimes make the gearbox last a little longer by replacing the gearbox oil with fresh stuff (80/90 or 85/90 Hypoid gear oil), and gentle shifting and rev-matching can help too.
As well as the stop-plate adjustement suggested by VW54, other causes of poor shifting are a worn out shift-rod bushing (inside the tunnel just behind the gear sitck) and/or a worn coupling under the back seat tunnel hatch.
If your gear stick has a sloppy feel to it one of both of those will probably be the cause - there are procedures on my web site below for fixing them.
The stop plate itself (under the gear shift) can wear out too. Have a look at the inner edge of the ramp on the right side of the hole - if the inner edge looks rubbed and has a sharpish corner where it was bent up rather than a rounded corner, replace the stop plate (they are cheap enough). Remember, when putting it back on the tunnel, "ramp to the right" is the right orientation.
Rob
Rob and Dave's aircooled VW pages
Repairs and maintenance for the home mechanic
http://www.vw-resource.com