@Marin. I believe there have been several changes in the way the decks were made on GBs. Your vintage should be a built up deck, with teak laid over deck beams.
That is incorrect. Fiberglass GBs do not have a solid glass deck and never did (I don't know about the currently manufactured models). We have one of the first batch of fiberglass GBs made and its deck is constructed exactly the same as the newer ones. There is a subdeck "sandwich" of fiberglass-marine ply-fiberglass with the layer of glass on top of the plywood about twice as thick as the fiberglass layer under the plywood. Deck beams were used on wood GBs, not on fiberglass GBs.
The deck planks are bedded and screwed to the subdeck using about 20 million screws with teak plugs on top of them.
Later GBs, by which I mean early to mid 2000s give or take, have teak decking that is glued to the fiberglass-plywood-fiberglass subdeck sandwich instead of being screwed to it. This practice was also adapted by higher-end manufacturers like Fleming, custom yacht makers, etc. Much better arrangement as there are no screws penetrating what otherwise is a watertight subdeck (other than the fasteners used for hardware mounted on the deck; cleats, stanchions, etc.).
This same subdeck construction was used in most of the boats similar to GBs--- CHB, Puget Trawler, Island Gypsy etc. They may have varied in material quality and thickness, but the configuration is the same.
This type of subdeck (including GB"s) can develop soft spots from moisture getting down under the decking, usually between the plank groove edges and seams that have pulled away from the wood on one side or the other of the groove. Once there, the moisture can then migrate down into the plywood core of the subdeck along the deck screws which penetrate through the upper layer of fiberglass and into the wood core. The moisture can then start the core to rotting.
The prevention of this is to regularly inspect the deck seams and replace any which have started to separate. This can be difficult to see so a pretty close inspection is required. Replacing a seam or a section of a seam is not rocket science and once one learns the proper technique and materials to use is actually quite easy.
This is one reason a teak deck should always be washed with salt water, not fresh. If there are leaks in the seams, deck hardware, etc. that let moisture into the subdeck core, the salt water is less encouraging of the formation of rot than fresh water. Of course, if it rains on the deck then it will be fresh water getting down into the core......
Depending on how they were put in the deck screws can penetrate all the way through the subdeck in which case moisture can penetrate all the way through to the inside of the boat if it gets under the planks. If the subdeck core starts to rot, the moisture passing down alongside the screw and into the interior of the boat will carry the evidence of the rot into the boat, hence the telltale brown stains and rot smell.