Alongside the rest of the Alliance, he returns as an antagonist in the series' spin off The Recruits.
Hallstrom is first described in the first story of the series Agent to Prisoner as a forty one year old man with fair skin, grey-ish hair and brown eyes. He wears a white Nehru jacket like a medical tunic, black trousers and black dress shoes.
In his first appearance, Gustav has a very minor role so his personality is not quite so developed. While he is fairly close to Sverre Aarrestad he is also close to Anders Hammarberg and despite the latter not being as impatient as the former, he is first shown trying to get onto the former's good side during a demonstration of an escape pod that he has developed.
There is still a sense of confidence about him as he watches the demonstration and becomes far more convincing.
The Cold War II series
Agent to Prisoner:
In the first story of the Cold War II series, Hallstrom is first shown with the Norwegian army general Sverre Aarrestad at the Honningsvåg Airport in Norway to demonstrate a new escape pod he has developed, the demonstration involves himself, Aarrestad and an un-named Finnish man. As Sverre watches through a pair of binoculars, Gustav gives clearance for a test plane to take off and watches as it does.
Time passes however and nothing seems to happen. Hallstrom's demonstration appears to be failing and general Aarestad is not impressed. That is, until the plane starts to dive and Gustav has him look which ultimately leaves him amazed: As the plane dives, part of the roof flies off and the ejector pod flies into the sky before descending into the water where its passengers are picked up by a submarine and the plane flies back under the mountain ready to be re-used.
Hallstrom later appears at a meeting with Scandinavian Alliance officials with regards to the kidnapping of Emma Irwin and Benjamin Simmons, two applicants for the E.I.N. the kidnapping of which is successful as shown when he is shown the video of Emma and Benjamin in captivity at the hands of Alliance members.