No. But yes. But no. It’s complicated, let me explain.
Greenwich Mean Time (GMT) is the local mean time in Greenwich, London. Local indicates it tracks the position of the Sun on the sky.1 However, because Earth’s rotation speed varies, a second according to GMT has different lengths on different days.
Meanwhile, Coordinated Universal Time (UTC2) is based on atomic clocks which guarantee constant length of a second. Unfortunately, Earth refuses to conform to human standards. To account for that, leap seconds are occasionally applied to UTC. As necessary, one second can be added (resulting in time 23:59:60) or removed (resulting in day ending at 23:59:58) at the end of June or December.3
While GMT and UTC use different methods for tracking time and adjusting to the Earth’s irregular rotation, they are synchronised to within 0.9 s and for everyday purposes they are the same.
Unfortunately things can get more convoluted. Someone may incorrectly use GMT to refer to time zone in London which is UTC+1 during daylight saving time. Furthermore, throughout history there were additional conflicting definitions of GMT.
Conclusion
All in all, it’s best to use UTC to avoid ambiguity.