I'm going to add support to what Grant suggested: if at all possible replace the front forks/ wheel/ brake with one off a R100/R80 GS (easiest) or another suitable donor.
I found that pounding on washboard caused the top fork bolts to loosen up and leak. I had to shim them to keep them tight, and add a o-ring to keep them from leaking. Despite this, they are still poorly dampened and flexible, and the brakes are pretty bad as well. You can fix the brakes, but I'd advise a fork change first. Upgrading to a R100/R80 fork makes a big difference, and the front brake is tolerable. This upgrade along with a new rear shock deals with 90% of the major issues with the G/S.
Apparently certain Showa forks will bolt right on as well, although most of these are off smaller, lighter bikes so more modifications are required to make them work.