The first story, Gallifrey: Reborn by Gary Hopkins, brings us to a world where Time Lords are mercenary and ultra-capitalist, trading regenerations with each other; and Mary Tamm plays a Romana who never left with the Doctor, while Leela is celebrated as the Doctor's companion during the quest for the Key to Time to the point of being made President with K9 as her Castellan. It all ends in disaster of course. Conrad Westmaas, who several years ago had played audio companion C'rizz for Big Finish, is much more comfortable here as Romana's rising young Time Lord son. We had Mary Tamm and Lalla Ward spark against each other as the two Romanas in earlier Gallifrey plays, but this takes a gloomier look at the dynamic.
Gallifrey: Disassembled</b> by Justin Richards brings us to a world where the Time Lords are actively interventionist in the time lines and, frankly, evil: the power-hungry President Romana, aided by Leela, her Interrogator-General, keeps a tight grip on matters; and what of the character played by Colin Baker who responds to, but doesn't like being called, the name "Doctor"? There's a lot of continuity-style speculation also about the true relationship between Braxiatel and the Doctor, and a rather odd rule of Time about meeting someone who you killed in a different timestream (never heard of this before, and it's flatly contradicted in the fourth play in the series), which weakens the promising start of the story, but it is still fun.
Gallifrey: Annihilation by Gary Russell and Scott Handcock takes us to a devastated, depopulated Gallifrey, where one of the oldest Time Lord conflicts, resolved æons ago in our more familiar timeline, is still continuing. One of the disadvantages of paying for the downloads but not the CDs is that you miss out on the cast lists for each, and I found myself trying to work out why Magistrix Borusa sounded so familiar: turns out she is actually Katy Manning, playing what she describes as her own voice with no Jo Grant perkiness or Iris Wildthyme bawdiness, and doing it rather well. Other guest stars include Geoffrey Beevors as Lord Prydon, who may or may not be an alternate and altered Master; Wendy Padbury's daughter Charlie Hayes; and co-author and director Gary Russell, in a rare appearance as performing artist (though of course we should remember that his career began as Dick of the Famous Five in 1978).
Gallifrey: Forever again starts with an alternate Romana who is a ruthless ruler of Gallifrey, but she is assassinated in the first couple of minutes, and 'our' Romana then has to work out what is going on in this slave-owning, non-time-travelling society, posing as its president, under the vicious supervision of the alternate Narvin who works out pretty quickly what is going on. The voice from the past this time is Carole Ann Ford, playing one of the slave labourers working on the Eye of Harmony (and John Leeson gets to do a different funny voice, a couple of other characters from the previous Gallifrey series return in alternate form). We end up with a situation which liberates Leela either to go into the three Companion Chronicle audios by Nigel Fairs which kill her off, or else for more adventures first.
Author David Wise, I note with interest, is one of only two contributors to the Doctor Who franchise to have also written an episode of televised Star Trek, co-writing the 1974 animated episode "How Sharper Than a Serpent's Tooth", which became the first and only episode of so-called "classic era" Star Trek to win an Emmy Award. (The other such writer is dduane, who co-wrote the 1987 TNG episode "Where No One Has Gone Before" and has had at least three Who short stories published.)
All in all, I think the four audios are relatively approachable considering that they come after three series of a spinoff line of plays. Lalla Ward and Louise Jameson, of course, carry it, greatly helped by John Leeson and in the first two plays by Miles Richardson's Braxiatel. But I think any fan with a vague knowledge of the standard Whoniverse timeline of Gallifrey should be able to enjoy these. See also reviews by Andrew Hickey and jeanne_dark here and here.