Biggles and the Gun Runners featured a number of actual aircraft types such as the Douglas DC-4 and the Lockheed Constellation. However, Count Alexander Stavropulos also used something called the Courier, which was described as an American aircraft, with twin engines and capable of carrying 6 passengers. In the book, the Courier was used primarily as an executive transport.
The Courier is, of course, a fictional name. But what aircraft might it have been based on? Whatever it may have been, it was not the Airspeed Courier, which was British and only had one engine. Only one Airspeed Courier out of 16 built survived the war and that was scrapped in 1947.
Since the Count was concerned with economy (his other aircraft, the Constellations, were cast offs bought cheap from an airline which was disposing of them), we can assume he wouldn't have used the modern executive aircraft recently developed at the time of the book, such as the Beechcraft King Air or even Biggles' own Swearingen Merlin II. Looking to fairly old but still usable aircraft types, we find, in fact, two aircraft which fit the description in the text almost perfectly: the Beechcraft Model 18 and the Lockheed Model 12 Electra Junior. In, fact, the Beechcraft 18 was, at the time of the book, regarded as the pre-eminent executive transport. It had two engines and seated 6 passengers just as the text says. More than 9,000 Model 18s were built and they saw ubiquitous service. Likewise, the highly similar Model 12 Electra Junior also had two engines and could seat 6 passengers and is another plausible candidate. Why Johns did not simply use these actual types must remain a matter of conjecture.