This gets asked with tedious regularity. At present, giffgaff seem to have no immediate intention of introducing this.
Remember that giffgaff is the budget brand owned by O2. At the moment, Wifi calling is a premium service, so if you want it you have to use the main O2 network and pay a premium. This will change once wifi calling becomes sufficiently widely adopted that giffgaff lose market share and hence profitability as a result of not offering it, or when they think that it will gain them sufficient additional (non O2) customers to make it profitable.
I don't know when that will be, but probably not for a while yet!