Please update the timezone displayed in the Achievements bar to use GMT instead of UTC.
The idea of using UTC (as a "universal" time standard) is that it doesn't suggest an indigenous preference to being in any particular location on Earth. You may argue that calling it a "timezone" is incorrect wherever you found it, but that would be the bit to change (bug or feature request). But it doesn't make sense to switch to GMT simply to make the use of the word timezone more correct. GMT and UTC are not the same thing. Thank you for the suggestion.
I'm sorry, but your pedantry compass needs to be recalibrated. Time stamps on Stack Exchange are on the UTC time scale, not on the GMT time scale. It's pedantry, because the difference is less than one second at any time.
Stack Exchange servers, like pretty much anyone on the Internet, presumably set their clock using NTP. NTP distributes UTC, including leap second information. Strictly speaking I'm not sure what happens during a leap second, so it's possible that events that happen during a leap second are recorded one second too early, but most of the time, the time stamp indicates the UTC time.
UTC is a time scale defined by the ticking of an atomic clock. Computers on the Internet generally set their time by synchronizing their clock with another computer using the NTP protocol, which itself synchronizes its clock with another computer, until the synchronization reaches a computer that's connected to an atomic clock. Unless you're a physicist doing experiments that require a lot better than millisecond precision (which is roughly what you can achieve through NTP in practice), all of those atomic clocks are equivalent. The atomic clock defines TAI, which is a number of seconds since 1 January 1958. UTC is TAI and adds a notion of day, which is normally 86400 seconds long but is occasionally 1 second longer (or in principle one second shorter, but it hasn't happened yet), to keep the calendar day from deviating from the astronomical day.
GMT has a long history but became deprecated in scientific usage in the 1970s. These days, people who care about subsecond precision don't use GMT anymore, so there isn't a useful authoritative, current definition of GMT other than as a different name for UTC. The last formal definition of GMT was based on the trajectory of the sun in the sky above the Greenwhich observatory, which is difficult to replicate, hence the switch to different formal definitions such as TAI and UTC.
“GMT” remains in colloquial use as a synonym of UTC. For everyday applications, it's perfectly fine, since GMT is a good enough approximation of UTC. But as far as I know, the United Kingdom is not officially on GMT but on UTC like the rest of Europe. (A page on the Greenwhich observatory website states that “Though it has now been replaced by Coordinated Universal Time (UTC), GMT is still the legal time in Britain in the winter”, but I don't understand what that's supposed to mean, unless it assumes that GMT and UTC are synonyms and “replaced” only refers to terminology.) The Paris Observatory website gives a reference to a French law that defines the French time in relation to UTC, and as far as I know this is compatible with the European directives on time.
So UTC is not wrong. Either you're ok with equating UTC and GMT, and either term is equally correct; or you aren't ok with equating them, and GMT is wrong.
In terms of naming, “UTC” has many advantages. As I've just explained. It's also more inclusive. It's also less ambiguous, because there's quite a bit of software out there that uses “GMT” to mean whatever time it is in the UK (so they say “GMT” for BST while BST is in use, i.e. they say “GMT” while displaying a time in GMT+1), whereas I've never seen “UTC” misused in that way. The only argument I could see to use “GMT” is if it's significantly more familiar than “UTC”, but I doubt that.