Both Polyamide and Polyester synthetic fibres are very durable and abrasion-resistant and are designed to absorb but not retain moisture and thus are good for moisture transport to move moisture away from the body. Both fibres are stretchy and don't readily crease, but Polyester can be prone to pilling so combining the two fibres increases the garment's durability.
Polyamide (Nylon™) is soft to the touch, really tough and has the highest resistance to abrasion of all textile raw materials even when wet and is also very stretchy. Polyester has excellent return conduct properties thanks to its composition at a fibre level so retains its shape and is colourfast, as well as resistant to sweat and UV-rays.
Polyester has a high melting point so transfers can therefore be printed on it more easily and both fibres dry incredibly fast due to the fact the fibres are Hydrophobic (dislike water).
Because synthetic garments are usually a multiple fibre composition and not a naturally occurring fabric, it has no GSM (Grams per Square Metre) applied to it. Only clothing that is predominantly natural fibre can be considered as having a GSM weight such as Merino Wool or Cotton where weight is often used to indicate the fabric's "density"