Gyllenkrok served as a teacher at the Infantry Officer School (Infanteriofficersskolan, IOS) from 1928 to 1931, and he was an expert in the committee for arranging Sweden's defense against airstrikes between 1928 and 1930.

[3] He transferred as colonel to the reserve and later had various commands and other military assignments and, among other things, since 1925 prepared various regulations and instructions for the army, navy, coastal artillery and air force.

He belonged to the circle of officers, who with Ny militär tidskrift ("New Military Journal") as a body, worked on a renewal of the Swedish defence after 1925 disarmament.

He was one of the leading forces and most active collaborators in the creation of the book Antingen – eller ("Either – Or") (1930), together with, for example, Helge Jung.

Among other things, he has published Exempel på ett reglemente för infanteriets stridsutbildning (together with Martin Hanngren, 1923), Arméns krigsberedskap (1938), Tysk krigföring (1940), Synpunkter rörande utbildning (1942), Partisankrig (1943), Partisankrig i snö, skog och berg (1945) as well as Utan uppdrag i Moskva (1945).