GSM stands for grams per square metre. It is a measurement of how much yarn is packed into a square metre of cloth, and on a towel that is almost entirely a measurement of the pile (the loops standing up off the base weave. It is printed on nearly every towel sold, and it is almost always presented as a quality score. It is not one. It is a trade-off, and understanding which side of it you want is most of the work of buying a towel.
What actually changes as GSM rises
Two things move together, in opposite directions. More pile means more surface area, so the towel takes up more water and dries you in fewer passes. More pile also means more water held in the cloth afterwards, so it takes longer to dry out. There is no weight at which you get the first without the second, because they are the same property described from two ends.
A towel at 350 GSM will dry you if you work at it, and it will be dry on the rail in a few hours. A towel at 900 GSM will dry you in one pass and, in an ordinary bathroom without a heated rail, will still be perceptibly damp twenty-four hours later.
Where the useful range is
For an everyday bathroom towel, somewhere between 450 and 550 GSM is where most people land if they try several. It is heavy enough that the towel does its job in one go, and light enough that a normal rail in a normal room can dry it between one shower and the next day’s.
Above about 600 GSM you are buying a towel that requires infrastructure. A heated rail, an extractor fan, or a bathroom with a window that gets opened. Given those, a 650 GSM towel is a genuinely nicer object. Without them, it is the towel that starts smelling in month three, and no amount of hot washing fixes it, because the problem is not bacteria in the cloth so much as a cloth that is never given the chance to dry.
Below about 400 GSM you are into gym and travel territory. Those towels are not bad, they are optimised for a different problem) packing small and drying fast in a bag.
Why shops push high numbers
Because GSM is the only spec most towels carry, and a bigger number is easy to sell. It also feels right in a shop: you pick up a 900 GSM towel and it is dense and heavy and obviously more substantial than the one next to it. That handling test tells you nothing about the property that actually matters, which is how the towel behaves on day four in your bathroom.
What GSM does not tell you
Quite a lot. It says nothing about fibre length, so it cannot predict whether the towel will shed lint for a month. It says nothing about yarn twist, and a low-twist towel at 500 GSM will out-absorb a high-twist one at 600. It says nothing about whether the pile is single or double sided. And it says nothing about the finish, which is why a brand new towel of any weight repels water until it has been through the wash once.
If you are comparing two towels and only one number is on offer, GSM is better than nothing. But the honest version of the question is not how high the number is, it is whether your bathroom can dry it.
A practical way to decide
Look at where the towel will hang. If the rail is heated, or the room has an openable window and gets warm, you have the headroom for 600 GSM or more. If the bathroom is internal, unheated, or shared by three people using the same rail, stay at 500 and buy an extra towel instead of a heavier one. Two 500 GSM towels in rotation will always beat one 800 GSM towel that never gets dry.