Guilds Factions and Cults contains rules for creating various types of organisations and a metagame that allows you to play them as characters - as it were. I don't know personally but it sounds like the metagame rules may not be very coherent. Probably the most important element of the book is that it gives new rules of magic for cult worshippers. Namely:
rules that allow casting rune magic without needing to integrate a rune
a comprehensive new divine magic system based on pacts with gifts, geases, taboos, myths and so on.
The new magic rules fit Glorantha much better than the old so it's probably fair to say that GFC is pretty much a must-have for Gloranthan RQ.
GM's Handbook is a mixed bag of resources. It has a long list of NPCs that I've found useless, a comprehensive list of skill test results, rules for alchemy and drinking, updated combat and skill suggestions. It's sort of an open secret that Mongoose are working on a new edition of RQ which makes me think that the GM's Handbook might become out of date. Personally I haven't found the GM's Handbook to be all that useful. There's nothing specifically useful about the book for Glorantha.
Arms & Equipment is full of lists of equipment and arms, surprisingly. I'm not much of a gear-head so I can't say I've ever got much use out of it. It's handy if you want gear at your finger tips. The item modification rules seem sub-par and, as with many things that Mongoose does, don't always fit that well with the rest of the system.
So, for playing RQ in Glorantha, GFC is a definite, GM's Handbook probably not that relevant and A&E pretty marginal.
