There are really two Gregory Sallusts in this series. A couple of commentators have considered there being three but I go with just the two to make things simpler. Allow me to explain.
According to what I have read, the author, Dennis Wheatley created what was likely to be a one-off adventure novel. It was titled Black August and though written in 1934, it took place roughly 30 years in the future where a dystopic Great Britain was on the verge of collapse. As one of the people fleeing the cities to the countryside for survival, Gregory Sallust takes on a role of leader, albeit one with his own agenda and not a little self-interest. In his mid-to-late thirties, Sallust is a former reporter who knew that that life was ending so he packed up and left. This novel was a glimpse of a very bleak future and a satire of sorts on the type of man one had to be to survive.
About two years later, the author had an idea for an adventure taking place in his contemporary times, 1936, with Europe gearing up for war and spies and agent-provocateurs every where. He wanted a strong, resourceful hero to do the dashing about that the story contained and he created him in a handsome, sophisticated, former reporter about to start a new job as a troubleshooter for a band of rich English industrialists. He chose as that man in his mid-30's, veteran of WWI, Gregory Sallust.
So, two Gregory Sallusts, both in their 30s but one doing so in 1934 and the other in circa 1964. The future Sallust was often a every-man-for-himself kind of guy but the 30s Sallust was far more the heroic type. Both were, the author described, "egoists" but the 30s Sallust was a good guy which is not something one really could pin on the 60s version. Why the two versions? Who knows. Some have offered their takes on it but the author did not, as far as I know, ever explain. There is no doubt, though, that he really liked the name.
What is known is that 3 1/2 years later when the idea of a British Intelligence agent working behind the lines during the early days of WWII came about, he turned again to his obviously beloved Gregory Sallust (the good one). This relationship would prove a good one (and profitable) such that he released the next book in the now-a-series 6 months later and yer another 4 months after that. A year and a half would pass (Wheatley was busy in the real life war effort) and then another adventure with Sallust in the war came out. Each of these wartime adventures covered a successive period though all took place in the early years which is understandable because they were written in those very times.
For several years after the end of hostilities, Wheatley worked on other books and other characters, including his Napoleonic spy series, Roger Brook. He was also continuing his Duke de Richleau series and his work dubbed the Black Magic novels. It would be six years after his last Sallust book that he returned to that character, this time for an adventure far away from the dark alleys of wartorn Europe. The Island Where Time Stands Still takes Sallust, a bit older and wiser, in search across thousands of miles of Pacific Ocean to China and then back to find the true heir to a island Chinese kingdom. This is the third Gregory Sallust some talk about but I prefer to think of him as the second just a few years later.
Four years after Sallust's oriental sabbatical, the author returns with him to WWII and the heady years of that conflict with Traitor's Gate, an adventure that spans the years 40-45. Wheatley concludes his Sallust War Year series in 1964 with They Used Dark Forces which also built heavily on his Black Magic leanings.
Another four years pass before Sallust is called to duty again but this time it is some time after the War, probably some time after the Island adventure. A Sallust in his 60's but still in pretty good shape for his age, gets involved in a hunt for sunken treasure to help a young ruler better his people's lives. The White Witch of the South Seas is the last Sallust recorded adventure but a darn good yarn.
So the first Sallust is a scondrel who lives 30 years after the second Sallust who is a hero worth rooting for. And that second Sallust (or a third if you go with the other hypothesis) ages nicely and stays active years after the War ends.
In my opinion, the first book, which I number as '0', is alright but if you skip it until later, I would not blame you. The second book should not be missed as it really gets you knowing the character. And then the 7 War Year books should be read in order. Finally, feast on the two non-spy adventures.