GPS satellites transmits GPS time (week & seconds) and the UTC time (Coordinated Universal Time) to the GPS receiver. Local time is displayed either by manual setting of the timezone or automatically by using "gmaptz.img" to fetch the local timezone at your current position. Using "gmaptz.img" automatically is very practical if one travels east<->west and crossing time zone(s).
Originally Posted by max_clif
NMEA 0138 - GPS Fix Data Sentence (PGRMF):
Time zones present (from March 27, 2011):
The UTC time is very close to GMT but may differ in sub-seconds level and it also uses leap seconds.
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Extract from Wikipedia:
As the Earth's rotation continues to slow, positive leap seconds will be required more frequently. The long-term rate of change of LOD is approximately +1.7 ms per century. At the end of the 21st century LOD will be roughly 86,400.004 s, requiring leap seconds every 250 days. Over several centuries, the frequency of leap seconds will become problematic.