Greenwich Mean Time, abbreviated as GMT, is nowadays a time zone designation rather than a time standard. Its succesor, since 1972, has been UTC (Coordinated Universal Time). There is never more than one second difference between GMT and UTC.
In the UTC standard, there is a commitment to keep within 0.9 seconds of GMT, so that every few years a leap second is applied to UTC.
In the age of the internet, these small clock calibrations are conveniently distributed via NTP.