A new sychro set, from the dealer, as said. The amount of money that it will cost will honestly shock you right out of doing it.
There is a bandaid fix, that will only hide the fact...its Synchromesh friction modified transmission fluid - Your GM dealerships will carry it (and if not, they can order it). It is expensive, but a hell of a lot less than replacing sychros on a tranny that isnt really worth it (I assume sohc). Penzoil also makes the same exact fluid, but around here its really difficult (impossible) to find in auto store...maybe youre lucky and can get it. The penzoil stuff is a lot cheaper than the GM crap, and does the exact same thing.
Get the friction modified, or dont bother.
I did this in my tranny and it solved the slight grinding I had within 100miles. Now all my shifts are smooth and make no noise at all when shifting. The problem is NOT fixed...but I dont have to listen to it when I drive down the road.