GSM stands for grams per square metre and refers to the amount of cotton that has been used in a product. The more cotton that is used in a product, the higher the GSM. GSM in towels usually ranges from about 450gsm up to 700gsm. The higher the GSM, the thicker and heavier the product is. The GSM does not affect absorbency but a lower GSM towel will dry more quickly than that of a higher GSM.
Yarn is made by twisting or bonding fibres together to produce a long strand in a process called spinning. Yarn may contain a single type of fibre or blend of various types. For example - Cotton vs Polyester/Cotton blend. This can then be woven to create towels and other textiles.
A ply is a single layer or strand of yarn. Plying refers to the process in which 2 or more strands of yarn are twisted together to make a thicker, stronger thread. Single ply means that the product has been made by weaving single yarns together. Double or 2 ply means that 2 strands of yarn have been twisted together to make one thread which can then be woven into a product.