Alexa can make outgoing calls via giffgaff. I have set up the service for my mother, who can ask Alexa to call specific people I have entered in her Alexa contacts list. There's no extra charge for using Alexa -- it just appears on giffgaff as if you have dialled the call yourself, so the call will be free from a goodbag (as long as it's a standard rate UK number).
There's more information at
https://www.amazon.co.uk/gp/help/customer/display.html/ref=help_search_1-2?ie=UTF8&nodeId=GHS4XD76UNEWHUEB&qid=1590137220&sr=1-2 should help you set it up.
Unlike vodafone, Alexa Calling can only do outgoing calls and cannot currently be used to answer incoming calls. That's something that o2 would have to set up and only then might it also become available via giffgaff.