CRA indicates that gifts made in Year 5 by a but-for GRE can only be claimed before death or in Years 5 to 10

Where an estate which otherwise would still be a graduated rate estate but for the passage of 36 months from the date of death makes a charitable gift in Year 5 of the estate (i.e., within 60 months of the death as required by s. 118.1(5.1)), that gift can be claimed in the year of death of the deceased or in the preceding year (s. (c)(i)(C) of the “total charitable gifts definition”), or (under s. (c)(ii)(A)) in the year of the gift or in one of the five following years (i.e., in Years 5 to 10) - but cannot be claimed in any of Years 1 to 4.

Neal Armstrong. Summary of 7 October 2016 APFF Financial Strategies and Instruments Roundtable, Q.8 under s. 118.1(1) – total charitable gifts – s. (c)(ii).