The latest beta should now be returning the expected $gmt(N) and $gmt(format) results. If it isn't, please let me know. That said, I have found an issue with z/zz/zzz being offset by DST incorrectly, which has been fixed for the next version.
Regarding negative time values prior to unix epoch: I have implemented some drop-in replacements for the CRT time functions that use the Windows API time functions. These can handle dates from Windows epoch 1 January 1601. These seem to be working. That said, it took a long time for the current date/time identifiers to stabilize due to the quirks in the CRT/API functions in different versions of Windows. I will be creating a /command that allows you to toggle whether mIRC uses the drop-ins or not, for testing purposes. This should be in the next version.