GB's are good boats as long as they have been looked after. Don't get paranoid about leaks, they will have some leak issues and don't get put off GB's by my post. I'm illustrating that there are ways to sort through and find good 42 GB. It's worth paying more for one that has has teak pulled and re fiber-glassed deck. It's a $40,000 dollar job...
Suggest to start looking at boats in poor condition 1st. This gives you a free education of what to look for in better condition boats. After you do this, you will be greatly informed.
Get or rent inspection camera with at least 4 feet of extension and memory to store lots of pictures. Beware of low resolution cameras on some units.
Deck: Bounce with your knees looking for soft spots, especially aft of salon and cockpit.
On deck get knee pads on. CRAWL around deck checking integrity of deck moldings. These need to be re bedded after purchase for piece of mind and its not too hard to do.
Get into lazarette and poke into corners and transom, bottom of water tanks,
Lazarette hatch's on 42's are good installation, but check them out as they are not water tight and allow some water into lazarette.
Interior: Get your coveralls on, exploring every nook and cranny, pull every drawer out and look inside for signs of moisture.
You can see quite a bit of GB's deck or where water would leak from behind drawers.
Windows and doors are there own subject but careful inspection will tell you plenty. Most of the leaks are from dirty and plugged felts and drains. Window casing re bedding will need to be done. Your mileage will vary, it a big job...
Engine space: From outboard side of engines look up at decks and behind fuel tanks. Use camera to inspect tank bottoms and tops, also look up at deck...
Forward cabin inspect where deck meets cabin sides. Water usually shows up at rear of hanging closet on top behind windows. If its bad it starts leaking through inside molding at front of cabin and drips onto cushions / mattress.
Look in all floor hatches at plumbing, bonding wires and cleanliness of bilge.
Take a good sniff test looking for sewage, diesel and oil smells. Check all through hulls for operation and hose / hose clamp condition.
One other engineering weak point of GB's is use of dimensional lumber to reinforce cabins and to mount interior fittings. This becomes a problem under leaking windows if it goes on for years.
Once you find a good candidate then find knowledgeable surveyor, make sure he looks at deck bedding and give you through opinion.
Don't worry about engines at this stage. Mechanical survey will turn up problems if any. Figure on replacing all batteries within your 1st year of ownership unless you have them load tested as part of survey.
Don't be afraid of 42 GB's.
