UNIX timestamp always in GMT?
Technically, no.
Even though epoch time is the means elapsed seconds since 1/1/70 00:00:00 the real "GMT" (UTC) is not.
UTC time needed to be changed a few times to take in to account the slowing speed of the rotating earth.
As everybody wrote, most people use epoch at UTC.
You can read more in https://en.wikipedia.org/wiki/Unix_time.
UNIX timestamp (A.K.A. Unix's epoch) means elapsed seconds since January 1st 1970 00:00:00 UTC (Universal Time). So , if you need the time in a specific TimeZone, you should convert it.
Even though is technically possible, I would recommend alternative ways to get current time (or any other time), such as getdate that already considers local timezone before returning.