i think the title says it all but ask away if you need any more details.
Some sellers may only show a partial listing of compatible frequencies. GSM could be referring to 2G GSM. While 2G GSM is not compatible with Koodo's network, there may be other frequencies listed for 3G (HSPA) and 4G (LTE). Providing a make and model number would be helpful or you can look it up yourself as per Goran's suggestion.