Given, for historical reasons, that the prefix is based on the International Telecommunications Union call-sign list I would have expected it to be GU- for Guernsey and GJ- for Jersey
That's very unlikely to happen.
The hyphen isn't considered to be a differentiating feature in registrations (it's not used at all in ATC callsigns), so a GJ-xxx registration would be indistinguishable from the 600+ G-Jxxx registrations already issued to UK-registered aircraft, ditto the 250 or so G-Uxxx marks already allocated.