Answer by taking a quote from the following article:

Namdev, also transliterated as Namdeo and Namadeva, (traditionally, c. 1270 - c. 1350) was a poet-saint from Maharashtra, India who is significant to the Varkari sect of Hinduism. He is also venerated in Sikhism, as well as Hindu warrior-ascetic traditions such as the Dadupanthis and the Niranjani Sampraday that emerged in north India during the Islamic rule. The details of Namdev's life are unclear. He is the subject of many miracle-filled hagiographies composed centuries after he died.

The literary works of Namdev were influenced by Vaishnava philosophy and a belief in Vithoba. Along with the Jnanesvari, a sacred work of Jnanesvar, and of Bhakti movement teacher-writers such as Tukaram, the writings of Namdev form the basis of the beliefs held by the Varkari sect of Hinduism. He was thus among those responsible for disseminating the monotheistic Varkari faith that had emerged first in Karnataka in the mid-to-late 12th century and then spread to Pandharpur in Maharashtra.  Namdev and Jnanesvar used the Marathi language to convey their beliefs rather than using the traditional Sanskrit language that was essentially a buttress for the pre-eminence of the Brahmin priests. Namdev's style was to compose simply worded praise for Vithoba and to use a melodic device called samkirtana, both of which were accessible to common people. Shima Iwao says that "He taught that all can be saved equally, without regard to caste, through devotion (bhakti) to Vithoba" and that he greatly influenced groups of people who were forbidden by the Brahmin elite from studying the Vedas, such as women and members of the Shudra and untouchable communities.  The earliest anthological record of Namdev's works occurs in the Guru Granth Sahib, the Sikh scriptures compiled in 1604, although Novetzke notes that while the manuscript records of Namdev mostly date from the 17th and 18th centuries, there exists a manuscript from 1581 that presents a rarely recounted variant version of Namdev's Tirthavli, a Marathi-language autobiographical piece. It is evident that the Guru Granth record is an accurate rendition of what Namdev wrote: the oral tradition probably accounts significantly for the changes and additions that appear to have been made by that time. The numerous subsequently produced manuscripts also show variant texts and additions that are attributed to him. Of around 2500 abhangs that were credited to him and written in the Marathi language, perhaps only 600 - 700 are authentic. The surviving manuscripts are geographically dispersed and of uncertain provenance.

When did produce his works?
The earliest anthological record of Namdev's works occurs in the Guru Granth Sahib, the Sikh scriptures compiled in 1604,