Yes, they are taxable under Section 6(1)(f) of the Income Tax Act. Not sure what kind of plan your company has with Great West Life. Interpretation Bulletins are elaborations and rulings made by Canada Revenue to explain how they treat benefits and what medical expenses can be deducted to reduce taxable income. See Interpretation Bulletins IT-54, IT-85R2, or IT-428.
Which of the 3 categories below your Great West Life Plan falls under, I can't say. You may have to call CRA or Great West Life for clarification.
(i) a sickness or accident insurance plan,
(ii) a disability insurance plan,
(iii) an income maintenance insurance plan, or
(iii.1) a plan described in any of subparagraphs (i) to (iii) that is administered or provided by an employee life and health trust,
http://laws-lois.justice.gc.ca/eng/acts/I-3.3/page-3.html
Anyhow, the benefits are taxable just like CPP disability benefits, or any other wage loss plan like EI. BTW: Depending on the severity of your husband's condition, he might qualify for the Federal Disability Tax Credit if he has a listed condition.