Hawgwash-- GB36 tri-cabins made with the new molds that replaced the original molds in 1988 have a port door like the GB42. It's a little farther forward in the side of the cabin than the 42's.
The extra length of the GB42 is distributed among all three cabins. The most apparent difference between the two models is in the main cabin which is noticeably longer viewed from the outside and more roomy feeling when you're inside. This is helped by the extra width of the GB42.
The new molds also made the GB36 taller. So the headroom,particularly in the aft cabin, is better. I'm 6'2" and my head touches the headliner in the aft cabin of our 1973 boat. We chartered a 1991 boat before buying our own and I had plenty of headroom in the aft cabin.
The new molds also allow for a separate shower stall in the aft cabin so one doesn't have to have the combination head and shower used on most of the pre-1988 boats.
Because the post-88 Gb36s are taller (and a wee bit longer) the forward head, which on our boat is very small, is large enough to include a shower with a curtain surround.
There have been a number of interior variations in the GB36 over the years, some of them very clever. One of our favorites is a port-side settee in the main cabin that has an insert that fits across the port doorway and a removeable panel in the aft main cabin bulkhead that creates a full-length berth on the port side using the settee and the top of the port hanging closet in the aft cabin.
While the interior compartment dimensions are consistent within each version of the GB36 tri-cabin (called the "Classic" in GB nomenclature) there have been enough variations in the compartment layouts and features that just about the time you think you're safe in saying all post-88 boats are like this or all pre-88 boats are like that, you come across an exception.
Of all the now out-of-production GB models our favorite by far is the GB46 Classic, particularly the galley down configuration. That's a boat you can really do something with as the number of staterooms and the size of the main cabin provide a ton of flexibility. And the step-down aft cockpit with the transom door to the swimstep is brilliant. GB eliminated that feature toward the end of the GB46's production run in a desperate effort to reduce the production cost of the boat. Major mistake in our and everyone else's opinion we've talked to about this including the GB dealer in our harbor and while it's not the sole reason by any means, GB killed off the GB46 a few years later.