Grandma is one of the three skins released under the monthly Skin'O'Matic offer. It could be individually purchased with game coins in mid April 2018.
Grandma could be purchased in the Skins Shop under the ‘Skin-o-matic’ tab for 690 coins. This skin was available for a limited time and may not be available for purchase in the future.
Details of this skin may vary depending on location, platform or time.
Note: The Skin'O'Matic offer with the Grandma skin was released again for a limited time in late July 2019. The cost for Grandma remained the same at 690 coins.