Welcome to the community!
There is little published information regarding the decommissioning of the Rogers GSM or 2G network. While it has been widely rumoured that Rogers plans to decommission the 2G (GSM) network by 2018, there has been no official statement from Rogers to that fact.
Interestingly, the souce of that rumour -- Rogers' own description of their Network Coverage, replaced their earlier statement:
In addition to being first in Canada with LTE, 4G HSPA+ and HSPA networks, Rogers operates Canada's only nationwide 2G GSM/EDGE network, and has committed support until 2018.
The only national carrier offering combined GSM, GPRS, EDGE, 4G HSPA+, LTE and multi-path digital fibre network technologies.
~ taken from here. Note no mention of 2018.
That said, I did find a possible timeline for the 2G sunset (see here).
Unfortunately, I haven't been able to find a more definitive date for when the GSM network is to be decommissioned.
Hope this helps