This is a common sight during body disassembly and inspection. Water pooled up around the windshield and backlight, and over time it rusted through. GTOs are like most other cars of the ’60s in this regard. If you can see rust damage beyond the molding, there absolutely will be more hiding under it. To fix this problem, remove the windshield trim, windshield adhesive, and windshield. Once removed, you can see rust damage from years of invasive moisture. At this stage, you need to thoroughly clean out the area and remove the water, leaves, and whatever else is in there. Then media-blast the area to remove all the rust and and get down to the bare metal. In some cases, an entirely new windshield mounting channel needs to be fabricated.