Uncertain. Has been linked to uncial Latin G or Runic แ(/โ jโ /) and even to a cursive Ancient Greekฮ(X, โxiโ). Snรฆdal (2015; see references) favours the Latin origin, possibly invented after Wulfila.
Snรฆdal, Magnus, 'Gothic Contact with Latin: Gotica Parisina and Wulfila's Alphabet', in: Askedal, J.O. and H.F. Nielsen ed., Early Germanic Languages in Contact (Amsterdam 2015) pp. 91-108, especially 100-101.