This is a very good question and the information we have found seems to vary a bit. However the consensus, at least in BC is that GST is allowed to be charged on gift card purhases, but PST should not be charged. Again I would recommend checking with the CRA to see if that can give you more clarity for your particular circumstance.
In my experience, when buying a gift certificates directly from the company providing the services eg. A $50 voucher at a massage business, they may only charge you the face value of the voucher since you will be back to use their services and tax will be charged at that time. When buying gift cards at a store other than that one providing the service, it is likely that the re-selling company will charge you some tax on the sale.