Teak sole and gunwale/covering boards new install

To put it bluntly, it's a very bad one. Sorry, but you asked...

Is teak nice to look at? Of course!
BUT, it's also expensive, slippery when wet, awfully hot in strong sunshine, and very demanding.
Besides, it's a disposable item, particularly when glued on top of fiberglass, rather than used as a structural component on wooden boats.
And having owned a wholly wooden built trawler for 17 years, with acres of structural teak decks (3/4" thick), I know what I'm talking about.

Let me put it this way: as I understand, you are talking of a 22 years old GRP boat, whose deck (I guess) needs nothing at all, if it weren't for your aesthetic wishes - correct?
Well, the moment you put teak on it, you can wave good bye to such fortunate situation.
At best, you'll need to replace it in a dozen of years.
And at worse (depending on how well the job is done, the teak selection, its seasoning, and another dozen of other factors), you might have moisture in between the teak and the GRP after a couple of years, if not even earlier.

Bottom line, I couldn't summarize my view better than Capt Ralph already did: I love wood on other boats.

All that said, if you'll decide to go ahead with the idea, I can only add two final words: good luck!

I didn't notice this bit, when I guessed that your GRP deck is still fine.
But what do you mean by that, exactly?

I agree that leaving teak natural on thick solid boards is a disaster waiting to happen. And iroko is even worse, BTW.
On the other hand, having tried to discourage Beau as much as I could, if he really aims at aesthetic over practicality, varnished is even better than oiled.
A proper maintenance PITA, though - below is the reason why I know.
Brought back to bare wood every 5/6 years, and refreshed with two hands every given season!

You're talking of caulked planks, I suppose? Nothing wrong in leaving them natural.
It's with the border planks and covering boards, that you'd better protect them.
Also in my previous photo, the caulked teak planks forming the deck sole have always been left natural.
Not a good idea with big chunks of teak used for covering boards, though.
Over time (depending mostly on thermal excursion and sun strength in your boating grounds), they become prone to splitting/cracking, if neither oiled nor varnished regularly.

That aside, I struggle to understand what you meant by Jarrett Bay refit with glass removal.
I never saw a Jarrett Bay in flesh, but AFAIK their boats are built with cold-molded wooden hulls, aren't they?
And even without knowing the details of JB technique, the decks of such boats are simply impossible to replicate, on a GRP hull.
I mean, even if you would cut and remove your GRP cockpit sole completely, and ask JB to rebuild it from scratch, they would have no way to mate it with your hull in the same way as they can do with theirs, where the deck becomes concurrent with the rest of the hull structure.
In fact, I'm pretty sure that they would advise you against that!

For covering a GRP deck with teak, the technique described by Capt.J is indeed the more widely used nowadays, also by boatbuilders.
Who by the way outsource the production of these pre-built to measure decks to companies focused on woodworking as such, i.e. not only for boats.
But I would be very surprised if JB would use that system, because I expect them to use the traditional method instead, with single teak planks laid on beams and caulked afterwards, leaving aside the details of what normally goes in between.

Just be aware that it's a non-return decision: if for any reason you wouldn't be happy with the result, restoring the GRP deck as you know it would be a mission impossible.