Starting in 1935, he attended the Marian Fathers Secondary School in Bielany in Warsaw. During the German occupation, he lived in Warsaw. He continued his education through secret high school classes organised by the Marian Fathers in a legal Municipal Road School. In the spring of 1941, he passed his Matura exams, after which he began studying Polish philology at the secret classes of the University of Warsaw. At the end of 1941, he became a member of the underground organisation Konfederacja Narodu (Confederation of the Nation). From 1942 he was a reporter for its magazine Nowa Polska – Wiadomości Codzienne (New Poland – Daily News).
He was active in the circle of young artists gathered around the magazine Sztuka i Naród (Art and the Nation) and the so-called Cultural Movement. He made his debut in 1942 in Sztuka i Naród with the poem Wczorajszemu (To Yesterdays). In 1942, he became a member of the editorial office; in November 1943, after the death of his predecessor, Andrzej Trzebiński, he became the editor. There, he published his poems, prose fragments, articles and reviews (under the pseudonyms Karol Topornicki, Roman Oścień and also anonymously).
He was a member of the Home Army and cooperated with the literary paper of the Rój Department of Mobilisation Propaganda of the Home Army Information and Propaganda Bureau. He participated in underground literary evenings, during which he presented his works. In 1942, he received the second prize in the ‘Biuletyn Informacyjny’ competition for a soldier's march for his poem Uderzenie (Strike). In 1943 he won the ‘Kultura Jutra’ prize for his poems Śpiew Murów (Singing of the Walls) and Rapsod o Warszawie (Rhapsody About Warsaw).
He took part in the demonstration organised by W. Bojarski to lay flowers at the Nicolaus Copernicus Monument on 25th May 1943 on the 400th anniversary of the scientist’s death (Bojarski perished after being shot by German soldiers while laying the flowers). In 1943, he gave up his paid work and studies, devoting himself entirely to underground work. In the spring of 1943, he participated in the Press Courses of Military Rapporteurs organised by the Home Army Information and Propaganda Bureau. After the beginning of the uprising, he fought in the Old Town in the assault group of Lieutenant Ryszard (Jerzy Bondorowski). He was killed either on 16th August 1944 at Przejazd 1/3 Street or on 22nd August 1944 in the Arsenal.
He was affiliated with Konfederacja Narodu, an extreme right-wing organisation, which continued the pre-war activities of the nationalist Obóz Narodowo-Radykalny, or ONR (National Radical Camp). In his journalism, in Sztuka i Naród, he expressed views in line with the organisation and the magazine – a negation of the value of interwar art (from which he excluded Czechowicz and Miłosz) and emigration art, emphasising the historical role of Poland and agitating in favour of battle-themed poetry. However, the view he presented was more literary than ideological and he showed some conciliatory tendencies towards writers of other political orientations, as well as some signs of a departure from the totalistic concept of literature and culture proclaimed by Konfederacja Narodu.
Gajcy's poetic work is concentrated in the poetry books Widma (Phantoms), Grom Powszedni (published in the underground), poems and short stories published in Sztuka i Naród and works left in the form of manuscripts, such as Misterium Niedzielne, presenting the tense atmosphere of the last months of 1939 in the form of a folk story and the drama Homer i Orchidea (Homer and Orchid). He started with Tyrtaeic poetry, in accordance with Sztuka i Naród’s style, but in his mature work he developed his own vision of poetry.
Referring to catastrophism, mainly of the Second Avant-Garde, he presented, however, his own vision – one that was saturated with negativity, emphasising the image of the apocalyptic macabre, and introducing elements of grotesque. He showed reality in vivid and symbolic images, often using the convention of dreams to combine realistic and fairy-tale elements, and, aside from poems, he reached for forms of ballads, fairy tales, carols and songs. In the volume Grom Powszedni (Our Daily Thunder), the leading motif of the poems became the idea of the mystical sense of sacrifice in the name of love for people and the homeland, and the key issues were taming death and the choice between hatred and love, ultimately made in favour of love.
Tadeusz Gajcy is considered to be the most outstanding poet of the Generation of Columbuses, alongside Krzysztof Kamil Baczyński.
In 2009, the Warsaw Uprising Museum released a CD entitled Gajcy!, with works by contemporary musicians set to the poet's texts.
In the same year he was posthumously awarded the Commander's Cross of the Order of Polonia Restituta.