Gore-Tex fabric is best known for its use in protective, yet breathable, rainwear.
Gore defined "breathable" as passing water vapour about 1/20th as fast as uncoated fabric, the same as most urethane coatings.
Gore-Tex was promoted as preventing overheat solely due to it's "breathability". Much of the rainwear made for big spenders is promoted as "breathable".
Note that Gore requires manufacturers of Gore-Tex to put extra ventilation in their rain clothes, such as "pit zips". They also require a durable water repellant finish on exterior fabric so rain can never reach the Gore-Tex film. The fabric is also relatively heavy.
Most users praise Gore-Tex only for warmth, if it is snugly closed at neck, wrists and waist, so cold air can't flow through it.
The simplest sort of rain wear is a two layer sandwich. The outer layer is typically nylon or polyester and provides strength. The inner one is polyurethane (abbreviated: PU), and provides water resistance, at the cost of breathability.
Early Gore-Tex fabric replaced the inner layer of PU with a thin, porous fluoropolymer membrane (Teflon) coating that is bonded to a fabric. This membrane had about 1.4 billion pores per square centimeter. Each pore is approximately 1/20,000 the size of a water droplet, making it impenetrable to liquid water while still allowing the more autonomous water vapour molecules to pass through.
Both wear and cleaning will reduce the performance of Gore-Tex clothes by wearing away the Durable Water Repellent (DWR) treatment on the surface of the fabric. The DWR prevents the face fabric from becoming wet and thus reducing breathability. The DWR can be reinvigorated by tumble drying the garment or ironing on a low setting.
Gore requires that all garments made from their material have taping over the seams, to eliminate leaks. Gore's sister product, Windstopper, is similar to Gore-Tex in being windproof and breathable, but has ability to stretch and is not waterproof.
The Gore naming system does not imply specific technology or material but instead specific set of performance characteristics.
Save your money for simpler and lighter fabrics
which often work just as well.