Bhagat Jaidev Ji is the earliest contributor to the Guru Granth Sahib Ji by date. He was a Bengali Sanskrit poet of the twelfth century, at the court of Lakshmana Sena, and author of the Gita Govinda.
Two of his compositions are in the volume, in Gujri and Maru, and they are not in Sanskrit. They are in an early Apabhramsa or proto-Bengali, and they are among the most linguistically difficult passages in the scripture.
The Gita Govinda itself, an erotic devotional cycle on Krishna and Radha, is not in the volume and has no place in Sikh practice. The two hymns are on the discipline of the mind and the futility of ritual.
A twelfth-century Bengali court poet is the far edge of the volume's reach, both in time and in geography, and he was put there by an editor working at Amritsar in 1604 with no reason to include him other than the text.
Two of his hymns are in the Guru Granth Sahib Ji, in Gujri and Maru. He is the earliest of the Bhagats by date and the only Bengali among them.
He is the author of the Gita Govinda in Sanskrit, one of the most influential poems in Indian literature and entirely different in manner from the two hymns in the volume.
Those two hymns are in an old Hindavi rather than Sanskrit, and whether they are by the same man is one of the standing questions about the volume's sources.