GSM tells you how many grams one square metre of fabric weighs. With towels this number is talked about a lot, and the usual assumption is "the heavier, the more luxurious". That is partly true.
A towel of 600 grams or more really does feel full and enveloping. But the same towel holds more water and takes longer to dry. In a humid bathroom, especially one with poor ventilation, a towel that is used again before it has fully dried will eventually develop that unpleasant musty smell. For everyday use, 450 to 550 grams strikes a better balance between drying you and drying itself in most homes.
One more thing: weight alone does not mean quality. A towel woven thick and loose from poor yarn can be heavy too. Weight needs to be judged together with how the fabric feels in the hand. It is a clue, not a verdict.