Different Time Definitions
UTC ("Universal Time Coordinated") is a time scale derived from TAI but adjusted with leap seconds in coordination (thus the "c") with UT1. In other words, UTC is designed to keep an, essentially, unvarying atomic time scale (TAI ) relevant to an imperfectly rotating Earth (measured by UT1).
TAI ("International Atomic Time") is a time scale based on an international ensemble of atomic clocks.
UT1 ("Universal Time One") is an astronomy-based time scale traditionally derived by combining UT0 observations collected from worldwide locations.
UT0 (“Universal Time Zero”) is a geographically local, astronomically measured, time scale.
GMT ("Greenwich Mean Time") is a version of UT0 as traditionally measured from Greenwich, England. Leap seconds are irrelevant to true GMT because GMT is astronomically derived.
GPS Time is an atomic-based time scale maintained by the U.S. Naval Observatory. Commonly, GPS receivers convert "GPS Time" to UTC by means of additional information encoded in the signal stream.
UTC is the modern equivalent of GMT in that it presents a worldwide and not-seasonally-changing time stream. They are encoded in the same format.
UTC and GMT differ by, practically, nothing. Moreover, many organizations, such as the BBC, which claim to report GMT are actually reporting time derived from an encoded radio or GPS source. In other words, they report UTC - sometimes even UTC derived from GPS Time.