Alas, there is no point in referring to the current results from $gmt(N). $gmt(N) is not catered for in any way in all previous versions of mIRC. It should have returned an error.
If I understand correctly, what you are requesting is for $gmt(N) to return the same result as $asctime(N). This will be in the next version.
mIRC uses CRT time functions in hundreds of routines to perform time calculations. These functions cannot handle anything before unix epoch.