I own 1985 GB36 #721. I've owned her for 16 years and do nearly all my own work whether it be annual maintenance or significant upgrades. In other words, I've been all over the boat many times. My areas of biggest concern:
1. Check the fuel tanks. They are prone to rust primarily on the tops near & around the fuel fill pipe. This is due to leaks around the fuel fill in the deck. New tanks are a big job and expensive. This is probably the biggest gotcha in a GB! I replaced mine and the tanks & new hardware alone was over $6K ... my labor was free.
2. Check for leaks all around the joint between the deck house and deck. There's a piece of teak trim that is bedded in that corner and after 30+ years the bedding compound tends to dry out and allow leaks. A likely spot to see a leak is inside the forward cabin hanging locker and around the aft cabin companionway. It's a pita fix but it's not hard or expensive.
3. Inspect the decks very carefully. Some owners tend to scrub the decks too often and it wears out the teak long before it should be worn out. Look for the seam compound standing proud, popped bungs, wet spots, and soft spots. The decks in my boat are original and although they need a little maintenance every year they do not leak. Pull open the lazarette hatches and feel the weight. These tend to leak and get quite heavy.
4. Be sure all windows slide easily. If not look for degradation of the felt lined tracks. If they are all gummed up then it's also likely that the drains are clogged which causes leaks into the deckhouse walls. Sometimes you can see stains from the leaks sometimes not.
5. Check the sheaves in the steering system. The sheaves can show unusual wear and need to be replaced, something that should be done 'cause a failure is a failure in steering!
BTW, I love my boat! It's one in a long line of 'em and it's definitely been my favorite. Comfortable, practical, well built and my wife feels safe onboard.