What is GSM?

What is a GSM and is it right to compare only on the basis of GSM? How Important is a GSM in the evaluation of a T-shirt? Its been well over 25 years for me in the business of manufacturing t-shirts and I have never understood why clients equate high GSM with better quality. After all GSM ( Grams per Square Meter) is nothing but the thickness of fabric. A thick t-shirt does not necessarily mean the same is good or will last longer. In fact, lower the quality of yarn the thicker will the garment be. Yarns are named as 4s, 7s, 10s, 16s, 20s, 25s, 30s, 40s, 60s, 80, 100s. Now as a manufacturer one has two options; he either uses a 20s yarn to make a t-shirt or uses a 40s double (two yarns of 40s intertwined) to create a t-shirt of the same GSM. 40s yarn is twice as much finer than its 20s counterpart and expensive too. Given the fact that GSM is one of the main parameters in comparing quotations, some manufacturers skip an entire process. Ideally after the grey fabric (raw fabric) is knitted, it is semi-bleached -a process known as RFD (ready for dyeing) and then dyed. Bleaching reduces the GSM (thickness) of the fabric thus making it lighter. For instance, a 220gsm fabric will almost become a 250 gsm if this process is skipped. Nevertheless, if the process of RFD has not been done, in a couple of washes the t-shirt will fade and lose its sheen. Therefore, GSM is not the right parameter to evaluate the quality of any t-shirt. Before bulk buying always ask for a sample and use it. If it looks good post wash, then one could presume the t-shirt is well made.