Pardon my rambling here. Huge topic.
First let me say, the material is dated, and needs pruning, but also great stuff!
Frankly I feel we should all stay in what the G.D. calls the "lower grades" for a pretty long time, until our personal hash gets settled. More on why below.
In the 1980s the G.D. derived material was pretty ubiquitous. Every book was using their approach, the geometric symbols and names, along with the obligatory chakras. A lot of us Gen-X magician got started on Donald Michael Kraig's 'Modern Magick', which takes many cues from Regardie to adapt the lodge material for personal use.
In my opinion, the G.D. "Sphere of Sensation" is a sort of Chaos Magic / New Thought filter. It can lead to some awkward syntheses if used with other systems , and is "manfesting" are very particular astral-emotional "inner temple" as a place to work through aspects of your Nephesh.
While I still think the G.D. system along with the Crowley - Bennet  A.A. curriculum  are great place for  newcomers who need a structured training program,  just know you are then sort of like deep sea diving in the spirit world , but wrapped up in a very Masonic diving bell. Some things you see and feel  are only on the glass of your helmet.
For those here whose objective is to get to the grims, they needs lot of pruning and re-contextualization to sync up with the grimoric tradition. We had some weird mixes in the late 1990's. Aaron Leitch is a G.D officer and grimoire magical an balances the two quite well, it seems to me, but I think he keeps them separate.
There may be others making training courses by now, but Rob Rider Hill's Black Chap Book, while more PGM and Greco-Egytian flavored , with Helios and Hecate as your Resident Titans, took inspiration from the G.D. He's a fellow G.V. guy, and while there are no goetic instructions in the book, but his book shows how the materal can be adapted while loosening some of the lodge magic framework.
Why I think this very, even overly structured stuff still matters.
Stephen Skinner in a video said Mathers was very well versed in the grimoires, but kept that material mostly to the side.  I also agree with him that when these lodge-Magic Masonic guys write about "Magic" what they are writing about is "Mysticism". While he is technically accurate, the inner training of a magician is part of the process.
Unless you are working with spirits (thought-beings) who have been brought down, the spirits and forces come through our personal minds first. Cleaning up you inner world is a very good idea. So unless one wants to go through, say, the  Zen route, or the Catholic Mysticism route, the G.D. is a very good system for tackling those thorny personal emotional issues to develop a  degree  of  inner balance first, before calling in demons,  angels, and other intelligences.
Also potentially destabalizing are the huge Achetypal mythic god-patterns that can overwhelm your everyday personality and lead to ego inflation.  The lower grades help you stay humble and not get "the magic"  wrapped up with your personal ego-projection, fantasies, and wish fulfillment.