The German guys, some of whom worked at the Opel factory, simply said it's a design flaw. All cars have design flaws like that. GT's are full of them, especially drainage problem flaws. "The Turd" where the wire bundle comes out the top of that box above the fuse box is one example. Water draining off the windshield goes along the cowling trough, drips onto the wiring bundle, through the fuse box, to the floor. Oooops! They took a big chunk of putty and tried to fix the problem that way. The wires should have come out the side of that box. Water draining into the trough around the hood opening pools up in the trough at the front of the hood opening, causing rust. Water that gets under the windshield gasket pools up at the bottom of the windshield rubber opening. The rubber bulb that you step on to squirt your windshield washer juice gets rusty inside and the juice leaks out, soaks into the carpet, and rusts the floor. Water leaking behind the windshield gasket and The Turd also do this. Any water that gets into the rocker panels is trapped there. There are all sorts of places where water gets trapped in GT's.
The unibody concept is partially at fault for a lot of rust problems with our cars. There are few natural breaks between panels for water to drain out of. Our cars, like all cars, were never meant to last more than about 5 years. If they were made to last forever they'd never be able to sell new cars. Planned obsolescence was a term bandied about back in the day, especially with American cars.