This from years of reading about woodworking and wooden boats:
Old wooden boats had teak decking laid plank by plank on the transverse deck beams. Those were 1" to 1.25" thick boards, with a rabbet of at least 1/4" at the top, with room for a bond breaker and caulking.
Newer wood boats interposed a layer of plywood over the deck beams and use much thinner teak strips, saving costs. GB apparently did this. The plywood needed to be thick enough to stand on without deflection, so is likely at least 1/2". The teak boards still look the same on top, with a rabbet at the top of one side and room for both a bond breaker and caulking. Some were nailed, some screwed, dependent upon the standard processes at the yard at the time of the build.
Over the nails or screws, a plug would keep out the water, until scrubbing or chemicals wore the surface, including the teak plug, down to the head of the fastener, then the screw or nail could act as a conduit for water to get past the head of the fastener. This is much less a problem in wood construction than in FG, as the nail or screw grips better in wood, leaving less (to zero) room around the shank of the screw or nail for the passage of water. In FG construction the substrate is stripped away by the fastener, so keeping water from getting down past the bottom of the teak board is more important. Modern teak decks are neither screwed nor nailed, to avoid this issue altogether, but are simply glued together with adhesive and laid on a bed of adhesive.
Your GB decks thickness will be dependent upon the standards of the yard at the time of the build, and upon the care taken by the various owners over its 50 (?) year lifetime to date. If you have a photo, what location is it taken from? Can you get access to get a photo where your anchor raising foot switch penetrates the deck? Or any other location where a fuel or water fill fitting penetrates? Or even the edge where your lazarette hatches sit? Or the laz hatches themselves? Any or all of those locations will instruct as to thickness and some even for whether screwed or nailed.
