In the early years of public land mobile networks, it used to be important for the caller to know which mobile phone technology the person being called was using. Later, the acronym ‘GSM’ in front of a number revealed that the subscriber likely was able to receive SMS messages. In addition, call charges could vary depending on the technology used.
Now it is 2017, and you can still see phone numbers prefixed with ‘GSM’. The acronym also tags along in other contexts: as an example, at least one bank provides a ‘GSM service’ that allows customers to bank using their mobile phones. However, specifying the cellular standard used is no longer necessary, at least for the kind of general usage mentioned above. Worse, the ‘GSM’ label is often false, as it denotes a second-generation (2G) technology that is becoming obsolete. Present-day mobile phones typically implement 3G standards such as UMTS, and the even more advanced LTE service has been launched.
In short, when you publish your mobile number, abandon the ‘GSM’ prefix unless it really is necessary for some reason. You can use plain ‘mobile’, but even that may be unnecessary, as landline phones are becoming less common.