I just fixed up two '76 GPs to sell this spring. Yes they are definitely worth "fixing up." They are no lightweights, but smooth riding machines. After you fix it up, if you take a liking to it you should consider upgrading to alloy rims. The stock steel rims are heavy and don't brake well in wet conditions. I took the leather saddles off to keep for myself before I sold them. Many riders find leather saddles to be desirable, so you might want to try it out. Here are a couple of pictures of the GPs cleaned up:
