If by separating you mean the caulking has separated from the planks, the caulking can be replaced. If the grooves are too shallow in places, they can be regrooved. Bungs can be replaced. Lots of how to videos on YouTube.
If the planks are worn beyond repair in many spots, it may be time to look at replacement. I think you are underestimating what a quality teak deck replacement would cost. If going with fiberglass, you’ll need to build up the deck to replace the strength/stiffness of the removed planks.
I am with gsholz in this.
Your teak decks from GB are likely 1/2" thick, so it is very unlikely that they are worn down so far as to need more than a general tune up.
You can tune them up easily, for little $. Get Lifecaulk black sealant. That is easy to apply and will adhere to the existing caulk and to the adjacent teak, if both surfaces are clean and dry. Do the spots that have separated. If they remain wet for a long time after a rain, wait for a dry couple of days, then use a hair dryer to get the last of the moisture out. apply the lifecaulk and grind it down to level after it cures.
If you also have plug (bung) failures, either by standing proud of the rest of the surface, or missing, remove both the plug and the screw below it. fill the hole with epoxy and put in a new plug. You may need to drill the hole a little to get a round, smooth hole of sufficient depth to hold the new plug. In drilling that new hole, you will see how thick your deck boards are at that location. It will be thicker than you thought. Once the epoxy has set, grind the new plug level. Bob's your uncle!
I had a few spots that were bothering me this year. The above is what I did. About 2 dozen plugs replaced, all the screws in those spots jettisoned and new plugs installed. now, only a couple of weeks later, I can't find most of the places repaired and I am happy with my 39 yr old teak decks.
I did about 12" of separating seam repairs a couple of years ago and didn't need to do any more this time.
The screws were used for many years by deck tradesmen. Not any more, as they eventually figured out that the black gunk the decks are laid on eventually sets up and holds them in place forever. The screws were redundant after the gunk set up, so your replacements don't need screws.
