I thought someone with more expertise might answer you, but since they've not I'll give you my thoughts. At first glance I thought it looked Germanic but actually it seems it's just anglicised. I don't have a Scottish Gaelic dictionary handy (when googling I found the chapter of the book you refer to and she says she is Scottish) but there is an Irish Gaelic word 'geis' pl. 'geasa' which means 'spell' or 'curse'. i tried googling 'geasach' and this does indeed mean enchanting in Scottish Gaelic and 'geasadach' is the Scottish Gaelic name for a peacock which I would guess means something along the same lines.
In Gaelic 'g' is always hard, whereas 'd' is their 'j' sound. I'm not so good with Gaelic vowel sounds though, and since it's anglicised I'm not sure exactly how the original word would be pronounced. Maybe someone else can help.