No-core Gamow shell model (NCGSM) is applied to study selected well-bound and unbound states of helium isotopes. This model is formulated on the complex energy plane and, by using a complete Berggren ensemble, treats bound, resonant, and scattering states on equal footing. We use the density matrix renormalization group method to solve the many-body Schr\"odinger equation. To test the validity of our approach, we benchmarked the NCGSM results against Faddeev and Faddeev-Yakubovsky exact calculations for ${}^{3}$ H and ${}^{4}$ He nuclei. We also performed ab initio NCGSM calculations for the unstable nucleus ${}^{5}$ He and determined the ground-state energy and decay width, starting from a realistic N ${}^{3}$ LO chiral interaction.No-core Gamow shell model (NCGSM) is applied to study selected well-bound and unbound states of helium isotopes. This model is formulated on the complex energy plane and, by using a complete Berggren ensemble, treats bound, resonant, and scattering states on equal footing. We use the density matrix renormalization group method to solve the many-body Schr\"odinger equation. To test the validity of our approach, we benchmarked the NCGSM results against Faddeev and Faddeev-Yakubovsky exact calculations for ${}^{3}$H and ${}^{4}$He nuclei. We also performed ab initio NCGSM calculations for the unstable nucleus ${}^{5}$He and determined the ground-state energy and decay width, starting from a realistic N${}^{3}$LO chiral interaction.