You are talking apples and oranges.
A GSM stands for Global System for Mobile Communication, notice, the word “mobile”, that is the key difference, while the WAN, stands for Wide Area Network. In better understanding, the Internet.
So, since you have mobile devices, phones, tablets, iPads, and other items, they would be part of the GSM, which would then connect to the WAN.
You can have a WAN without the GSM, but, you cannot have a GSM without the WAN.