LocalH discovered a mention of Gamma 1 in Amazing Computing (1986: volume 1, issue 9, magazine page 87). It reads:
AmigaDOS 1.2 developer updates
AmigaDOS version 1.2, gamma 1, number 33.44 was shipped to developers in mid October. It was a seven disk set.
Some developers have reported strange lock-ups with this version, especially when initializing disks. Hopefully, the release version will be truly free of strange bugs.
This was the last free update to today's developers. Commodore-Amiga will be switching to a new developer support plan. Future updates will be available for a price. The details of this plan haven't been released yet, but the bug report sheets for the gamme 1 release off a dollar credit reward to developers who report true bugs in this version.