Uncertain. According to SnΓ¦dal (2015; see references), most likely an adapted Ancient GreekΞ(Th, βthetaβ), though links to Ξ¨(Ps, βpsiβ) have also been made.
Snædal, 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 99-100.