In Canada for a class of people who had to relinquish citizenship because it was not possible to have dual citizenship, there is a provision to reclaim that citizenship. Gilles' maternal grandmother relinquished Canadian citizenship during the defined period in order to become a US citizen.
Once citizenship is granted to Gilles' mother, Gilles can become a Canadian citizen through her mother. It takes a while for all the processing to occur -- the last time I looked normal processing for Canadian citizenship was 19 months; I don't know if this is on a separate path.
Because Gilles and Poirier train in Canada, Gilles also would be eligible to apply for Permanent Residency under the entrepreneurial class under the self-employed category for world-class athletes and take the same course towards citizenship that Weaver did, but that would take years without being expedited like Weaver's was. Since she is/will be eligible for citizenship through her mother, once her mother's is granted, that was the most direct route, since it skips the permanent residency part, which takes at least a year to process, and she may not have to have three physical years of residency before she can apply for Canadian citizenship.
ETA: At least that was the plan when there was a newspaper article published about it.