GMT is not (officially) used anymore. UT1 is based on the Earth's rotation within the ICRF (almost J2000) reference frame based on the positions of distant quasars. UT1 is not a steady time frame, e.g. if you measure UT1 at two times one second apart, it probably will differ by something other than one second. This is due to the fact that the Earth's rotation rate is constantly changing.
UTC is what people normally mean when they refer to GMT or Universal Time. It has a steady definition for a second based on TAI, and is adjusted by adding leap seconds to keep it within .9 seconds of UT1.
Every day the International Earth Rotation Service publishes values for UT1-UTC based on actual observations as well as other Earth Orientation Parameters. They are also responsible for determining and announcing in advance of when a leap second will be added.
UTC is what people normally mean when they refer to GMT or Universal Time. It has a steady definition for a second based on TAI, and is adjusted by adding leap seconds to keep it within .9 seconds of UT1.
Every day the International Earth Rotation Service publishes values for UT1-UTC based on actual observations as well as other Earth Orientation Parameters. They are also responsible for determining and announcing in advance of when a leap second will be added.
E.g.: https://www.iers.org/IERS/EN/DataProducts/EarthOrientationDa... See "finals.all IAU2000"