Am I right in assuming that the more GSM the firmer the mattress?

Hello, what a helpful website you have. I currently have a Smeaton Brothers mattress and its finally come to the end of its life! 15yrs money well spent. I'm very impressed with your mattress range and would like to know can I buy just the mattress. I'm so impressed by the knowledge you have given me to buy. It's now just choosing from your Artisan range for me. I'm now going on GSM as the main factor for my new bed. Would I be right in assuming the more GSM the firmer the mattress will be?