Eddie - You said your GWL LTD payments are TAXABLE, so the following statement applies to you and DOES NOT apply to anyone who receives non-taxable LTD payments (like Allyoops’ situation).
Since you already paid income tax on the GWL LTD payments you received, you will not pay income tax again on the CPPD backpayment that Service Canada sent directly to GWL in Nov/Dec 2017 (as per one of your other postings).
Here’s a detailed explanation from one of my earlier posts.
You will receive your T4A from GWL shortly. You will notice that Box 107 on this form will show the gross amount (i.e. amount before income tax deductions) of all LTD payments GWL paid to you in 2017 LESS the backdated CPPD payment GWL received on your behalf from Service Canada. Box 022 on the GWL T4A will show the full amount of income tax deducted by GWL from your LTD payments throughout the year.
To summarize, although the 2017 T4A(P) you received from Service Canada included your backdated payment in box 16, this amount will be offset by an an equivalent reduction shown in Box 107 on the T4A you will receive from GWL for 2017.
This should be a completely neutral situation for you.