As a boy, Gruffudd was one of the hostages taken by King John of England as a pledge for his father's continued good faith.

Llywelyn, however, had excluded him from the succession and had declared Dafydd, his son by his wife Joan, to be heir to the kingdom.

On his release he was again given lands, this time controlling much of the commotes of Llŷn, Ceri, Cyfeiliog, Mawddwy, Mochnant and Caereinion.

Gruffudd's wife, Senana[2] (possibly a daughter of Caradog ap Thomas of Anglesey), agreed to pay Henry 600 marks for the release of her husband and their eldest son, Owain, and to hand over her two youngest sons, Dafydd and Rhodri, to the King as hostages to ensure that she kept her part of the bargain.

According to several non-contemporary Welsh genealogical tracts, the mother of Llywelyn was Rhanullt, an otherwise unknown daughter of Rǫgnvaldr Guðrøðarson, King of the Isles.