His family had a legacy in working with marble, since both his uncle and his father, Ioannis Halepas, were decorators of churches and constructors of tomb memorials for cemeteries. The talent of Giannoulis manifested at a very young age, but upon finishing grade school, his father sent him off to Syros to work as an employee in a commercial store. After intense conflicts, his family moved to Athens in 1869. Giannoulis enrolled in the School of the Arts and studied under preeminent classicist sculptor Leonidas Drosis until 1872. His unique talent allowed him to graduate in half the usual time, while at the same time sweeping all the prizes in all the contests. In 1873, a scholarship granted to him by the Holy Foundation of the Evangelistria of Tinos allowed him to go to Munich and continue his studies at the Academy of Fine Arts under Max von Widnmann. His awards and prizes that kept flowing in were prognosticating a brilliant career. In 1875, however, for unknown reasons and in spite of the efforts and appeals of his teachers, his scholarship was interrupted. Giannoulis managed to stay in Munich for a certain period of time, thanks to the help from his friend, later a historian, G. Konstantinidis. But in 1876 he was forced to permanently relocate to Athens, where he opened up his own studio. During that time, he produced two of the most important works of the early stage of his artistic creation: "Satyr Playing with Eros" (1877), for the model of which he had been awarded a gold medal at an exhibition in Munich in 1875; and "Sleeping Woman" (1878) for the tomb of Sofia Afentaki in the First Cemetery of Athens. These works, as well as the others he produced during that same period, are characterized by their realistic rendering and superb application of the lessons of Classicism.
In 1878, he experienced the first symptoms of mental illness. Whether hereditary, as some researchers argue, or caused by a love gone wrong, or a manifestation of his highly sensitive psyche, this illness led to his suspending his work for some forty years and, in 1888, to his confinement in the psychiatric facility of Corfu, after being diagnosed as “suffering from dementia”. Following the death of his father in 1902, his mother, who had always been opposed to his confinement, singed him out of the psychiatric facility and brought him back to Tinos. It is not known whether, during that long period of his confinemnt, he had continued to work, even on a rudimentary scale. The only surviving proof that his creative juices were still flowing during the time he spent in the psychiatric facility is a miniscule terra-cotta head of a male figure that he produced while instiutionalized. This primitively produced figure, with its unrefined, partly distorted face, could perhaps be seen as his tragic self-portrait.