As far as I can figure out, these are the preamps for the GZ1, 2, 3, and 6, all of which, or matching replacements, should still be available.
GZ1 – EQ-7545R
GZ2 – LC-5 (out of stock but…)
GZ3 – e-T5 – can’t find this one online.
I strongly suspect that the LC-5 will fit the GZ2 and may fit the GZ-3.
GZ6 – MiSi Acoustic Trio (if fitted)