The Grin is one of the three skins released under the first part of the Creepy Christmas offer. It could be individually purchased with game coins in mid December 2020.
The Grin could be purchased in the Skins Shop under the 'Creepy Christmas' tab for 5500 coins. This skin was available for a limited time and may not be available for purchase in the future.
Details of this skin can vary depending on location, platform or time.