It is the 'first decan of taurus ruled by mercury', which is not quite the same as 'Mercury in Taurus'. That being said many, including for example Crowley, seem to have interpreted it as such. Easy way to remember them if you already know the astrological wheel is that they follow the days of the week round the circle of the zodiac. The GD additional meanings based on decans are from the Picatrix and Agrippa. According to Agrippa:
The first decan of taurus:
A naked man, an Archer,
Harvester or Husbandman, and goeth forth to sow, plough, build, people, and divide the earth, according to the rules of Geometry;
Compare GD 5 of Pentacles:
Material Trouble Loss of money or position. Trouble about material things. Labour, toil,
land cultivation; building, knowledge and acuteness of earthly things, poverty, carefulness, kindness; sometimes money regained after severe toil and labour. Unimaginative, harsh, stern, determined, obstinate.
The largely negative associations of the 5's is based on their following Levi's system in associating the numbers of the pips with the sefiroth of the ToL and thus 5's correspond with Geburah and interpreting such in a mostly negative sense [strength, judgement, severity, fear, the left of hand of god, associated with Mars and the angel of Mars Samael, the cross eyed devil and 1+2+3+4+5=15 the 'ides of the month', full moon, witches sabbath and the tarot Devil].
Full list of Agrippa decans with comparison of GD meanings can be found at Uri Raz's site here:
http://www.tarot.org.il/decans.html
Kwaw