Well, actually GD astrology IS for all intents and purposes medieval/Renaissance "7 planet" astrology, because much of it comes from Barrett's
Magus which is plagiarized Agrippa. Actually, for our purposes classical astrology is weirdly coherent and unified across Europe, North Africa, and South Asia until Theosophy lobotomizes it in the late 19th century.
The trouble is, the historical scholarship available to the GD at the time on astrology was sketchy. I'm convinced (though it's only my opinion) that this is the reason so much to the focus is on QBLH (
Thanks, M. Levi!) & Enochia (
Thanks, Mr. Casaubon!) in later GD-based writers. Having said that, at least one member, William Peck, was a sometime astrologer and Astronomer Royal of Scotland. His charts served Mathers as a source in a couple order papers, though nothing too central. Mathers' use of astrology was often muddled a bit, and absolutely relied on texts he had available to him.
For your purposes, the thing to keep in mind is that the GD material, howver you want to characterize its sources, is
based on some very old esoteric material... and the astrology contained therein, is old-fashioned, Ptolemaic, 7-planet, pre-Alan-Leo predicitive astrology. What Frawley calls
The Real Astrology. Just give all that modern sunsign stuff a skip; it's not gonna help much and will confuse you a lot. Took me forever to figure out why I couldn't make heads or tails of the GD astrological material, and then I stumbled onto a copy of the
Tetrabiblos and Lilly's
Christian Astrology... Did my head in, in the best possible way.
A good thing for you to look into as your looking at the Decans:
essential dignity. See if you can get a feel for which planets like to be where, and why. A lot of the Decans reflect a complex interrelationship of dignity and detriment. Then maybe take a look at the attributions of the planets. Warnock's Renaissance astrology site has tons of info colated here:
http://www.renaissanceastrology.com/planets.html
And keep repeating to yourself: the planets are the juice, the house is the serving, the zodiac is just the bottle.