GSM stands for grammes per square meter and it refers to the quality of the paper. Standard copy paper is normally 70 to 80gsm, good quality notebooks usually have paper 80 to 90gsm and high quality writing paper is normally 90gsm and above. However, it's not always about the GSM as the composition and coatings or the paper makes a huge difference too.
All the writing paper and notebooks we stock have fountain pen friendly paper.