Sikh theology holds two things about God at once, and holding both is the point rather than a tension to be resolved. God is nirguna, without attributes: unconditioned, unlimited, not describable by any quality that belongs to created things, and in that aspect beyond the reach of the mind entirely. God is also sarguna, with attributes: manifest in and through creation, pervading it, knowable in it. These are not two gods and not two stages. They are the one Reality described from outside and from within.
The consequence is a strong immanence held alongside a strong transcendence. Creation is not an illusion to be seen through and it is not separate from God; the Japji Sahib describes countless worlds and the divine order, hukam, running through all of them, and living in accordance with hukam rather than in self-will, haumai, is the whole of the spiritual task. It also explains why Sikhi is emphatically a householder tradition. There is no withdrawal from the world to find God, because God is not elsewhere; the Gurus were married, worked, and taught kirat karni, honest labour, vand chakna, sharing what one earns, and naam japna, remembrance of the Name, as one practice in three parts.
The vocabulary here overlaps with the Hindu sant poets among whom Guru Nanak moved, and some of those poets (Kabir, Ravidas, Namdev, Farid), are in the Guru Granth Sahib itself, their compositions included alongside the Gurus'. That inclusion is a real theological statement about where truth may be found. It is not an identification of the Sikh conception of God with a Hindu one, and Sikh writers have generally resisted having their scripture read as a branch of Vaishnava devotion. Where Hindu divine names appear in the Guru Granth Sahib (Ram, Hari, Gopal, Govind), they are used as names of the one formless Reality, not as references to the Puranic figures who bear them.