GMT is a timezone that is tied to a specific geography UTC is a time standard for which every other time in the world is based off of (i.e. relative to).
As long as you're going to be pedantic, this isn't correct either. Greenwich
*Mean* Time is not the same as the local civil time in Greenwich, London, UK. As you've noted, the actual civil time in Greenwich town may vary due to British Summer Time, and also the local noon can vary due to the equation of time, that is, differences due to the season. But Greenwich Mean Time always stays the same (it's the mean or average local noon at the specific location of the observatory), and is a former, outdated international standard, based on a quite well defined specific way of measuring the average time at that location. It's simply been
replaced by UTC, but they are the same concept in the sense that they provide a universal time standard for everyone else to base their time zones from. There are some differences in the calculation concept that make UTC more reliable, but it's still tied to the same geography of the zero longitude line (prime meridian), which runs through the Greenwich Observatory.
I've never understood why they would change GMT to UTC in any event.
It's really underlying technical reasons that's mostly of importance to astronomers and time geeks, or those who work in related technical fields that rely on very precise timing, such as satellite navigation etc. The summary is that GMT is a traditional method based on observations at the observatory, which are subject to tiny errors at times. As atomic clocks were invented in the mid-20th century and time now could be measured much more precisely, without the error introduced by observation, international technical conferences were held to answer small questions about how to deal with the improved ways of measuring time. This is where you get into things like the revised international definition of a second (based on the caesium atom instead of earth observation), leap seconds to account for the earth's spin slowing slightly every year, etc. By using "UT" or later "UTC" you are signifying that you're using a time source that's compliant to all the current international conventions and the formal definition of Coordinated Universal Time as defined by the International Telecommunication Union and various treaties that implement the definition per-country. But if you use "GMT" you're signifying the use of an older, traditional measurement based system, rather than modern precise-in-every-way atomic clock method.
Certainly, the aviation industry uses UTC at all times, especially when it comes to things like GNSS (satellite navigation/GPS) where this high level of precision *
matters* - microseconds can mean hundreds of metres. It's the reason Qantas and Virgin had no crash at Mildura on 18 June 2013, which could have been Australia's deadliest day in our aviation history, but you perhaps you never heard about it . GPS saved the day by allowing two planes to land safely in thick fog at Mildura despite being well below weather minimums. (Details:
Landing below minima due to fog involving Boeing 737s, VH-YIR and VH-VYK, Mildura Airport, Victoria on 18 June 2013 | ATSB )
So yes there is a difference, and it matters, but mostly for historical and technical reasons - in lay parlance GMT and UTC are equivalent, and both refer to the international standard time (not the actual civil time in town at Greenwich).
