Hi Lisa Perry 
 
Thanks for your question, 
 
I'm glad it's been of assistance for you, I hope I can be too! 
 
GSM doesn't determine the firmness IE to think the higher the GSM the firmer the mattress wouldn't be quite right. The GSM stands for gram per square meter. This can be of any material usually used in combination with multiple layers to build up the overall soft/medium/firm comfort layers that are different in each model of mattress. This is not to be confused with the spring tension. 
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If you are to have a look at the Artisan Luxury this has 4600GSM is 100% natural fibre and our softest mattress in the upper (comfort) layers. 
 
1 1200 GSM BRITISH FLEECE WOOL AND COTTON
2 HAIRPROOF CAMBRIC COVER
3 1200GSM PURE horsehair
4 500GSM PURE WOOL
5 1200GSM PURE HORSETAIL
6 500GSM PURE WOOL
 
These layers give the mattress its softer feel in the upper layers. 
 
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If you then look at the Artisan Bespoke this has 5100GSMis 100% natural fibre and our firmest mattress in the upper (comfort) layers. 
 
1 500GSM BRITISH WOOL
2 1200GSM BLENDED BRITISH FLEECE WOOL AND COTTON
3 HAIRPROOF CAMBRIC COVER
4 1200GSM PURE HORSETAIL
5 1200GSM HORSEHAIR
6 1000GSM WHITE CASHMERE INSULATOR
These layers are designed to be a firm mattress and then you would pair this with the spring tension based on your body weight.
 
I would advise taking into consideration your own sleeping habits, whether you are a back, side or front sleeper. A front sleeper should be on a firmer mattress whereas a side sleeper would need something slightly softer. 
 
I hope this helps, if you have any further questions please don't hesitate to ask