Say Y here if your RTC (Real Time Clock a.k.a. hardware clock)
stores the time in GMT (Greenwich Mean Time). Say N if your RTC
stores localtime.

It is in fact recommended to store GMT in your RTC, because then you
don't have to worry about daylight savings time changes. The only
reason not to use GMT in your RTC is if you also run a broken OS
that doesn't understand GMT.