It's really poorly phrased in the article but I assume what they meant is that LHR is in a time zone that sometimes observes GMT, and otherwise observes BST (British Summer Time) in the summer which is 8 hours ahead.

It's not exactly correct that SFO is 7 hours behind GMT because this isn't true for around half the year. It also isn't exactly true that LHR is in GMT.

Most of the year, SFO is actually 8 hours behind LHR. A few weeks are off due to different DST dates.

The author/editor has mentioned elsewhere in this thread that he was using dummy data (which contains mistakes). For this dummy December flight LHR would be in GMT (UTC+0) and not BST (UTC+1).

GMT doesn't observe summertime, but LHR does. So LHR (and the rest of Britain) is only in the GMT time zone for half the year.

GMT is very nearly the same thing as UTC. Think of it as a reference rather than timezone, but one that happens to be used by a few places half the time.

It's just easier to think of yourself in UTC/GMT all the time (known as Zulu time or /Z in the aviation world) and deal with the relative offsets later. Don't forget that US Daylight Saving has different start/end dates to the UK while you're at it too...