On 17 June 1941 he was arrested by the Nazi authorities. He was incarcerated until April 1942 at Møllergata 19, and from 24 April 1942 to the war's end on 8 May 1945 he was imprisoned at Grini concentration camp. In the summer of 1942 both Gabrielsen and Lindbäck-Larsen were part of the so-called Forest Gang () at Grini, which spent the summer days cutting down trees near the camp. The forestry work allowed easy access to food hidden in the woods by sympathizers, and messages could be smuggled in and out with little difficulty, making the Forest Gang the most popular assignment at Grini. During his time at Grini Gabrielsen also took part in secret debating meetings organized by journalists amongst the prisoners, where various aspects of the organizing of post-war Norway was discussed. Amongst the issues Gabrielsen discussed with fellow Grini inmates was the post-war reconstruction of Finnmark, most of the county having been laid waste during the German retreat in Northern Norway in 1944—1945. One important question with regards to the future reconstruction was whether or not the devastated areas should be rebuilt like they were organized before the war, or if they should be reformed with the future development in mind. With only the single exception of Finnkongkjeila in Gamvik the reconstruction ended up following the old pre-war population patterns.