Gore-tex is always waterproof, so you can't re-waterfroof it. What you are probably askng about is renewing the Durable Water Repellant (DWR) finish. That's the chenical applied to the outer surface of the garment that causes water to bead-up and roll off. It's critical to the performance of Gore-Tex. When DWR degrades, the outer fabric can become soaked with water (wet-out) which diminishes the ability of the Gore-Tex to breathe and makes the fabric cooler. As a result, your sweat vapor inside will condense on the inner side of the fabric making you feel cold and clammy. The fabric isn't leaking, it's your own sweat making you wet.
DWR is easily renewed. The first thing to try is machine washing and drying the garment (don't dry a dry-top in the dryer because it can screw up the latex gaskets). That will usually bring back the DWR. If that doesn't work, there are several good spray on or wash in DWR replacements. I personally like Nik Wax products. Renew the DWR and your Gore-Tex should be good to go.