The location and date of his birth are not known with certainty. It is likely that he was born in Ghent where he was baptized on 4 May 1623. However, there is also a record in Namur dated 10 June 1665, in which he is listed as a native of Overijse. An album with drawings mentions him as a native of Lille. It is not clear with whom he trained although some scholars mention the Antwerp painter, draftsman and printmaker Lucas van Uden as a possible teacher since some of Neyts’ landscape paintings reflect van Uden’s style, however there is no evidence for this presumed pupilage.
In 1643 he was living in Antwerp where he married Clara de la Porte, the couple had two daughters and a son. In 1647 he was registered in the Guild of Saint Luke in Antwerp as a master painter and engraver. In 1650 he spent some time in Dortrecht and in 1653 he may have made a trip to Spain as it is known he made plans to do so. From 1662 he was active in the Meuse valley around Namur and Huy where he made many studies of landscapes. In 1665 he is registered as a poorter of Namur where he enjoyed the patronage of the local monasteries, he also collaborated with Jacques Nicolaï who painted the figures in 18 landscapes commissioned by the Church of the convent of Croisiers in Namur. Neyts travelled widely throughout the Meuse region and portrayed its cities, steep-banked valleys, imposing castles, ruins, rivers and luxuriant forests. He probably worked in Lille in the late 1670s. On the basis of a drawing of Antwerp, he is believed to have returned to Antwerp around 1680. He remained there until his death around 1687, the year in which the register of the Guild of Saint Luke mentions the payment of his death debt. He was buried in the Antwerp Cathedral.
While in his early career Gillis Neyts produced some religious and mythological paintings, the vast majority of his later output was as a landscape artist. His subjects are either imaginary, idealised panoramas or topographical views of towns and villages and often placed figures in his landscapes to animate the foreground or the composition. Even his works on religious themes were landscapes with small devotional figures included. He also produced a number of harbour views with ships. Neyts was able to develop his landscapes with infinite variation in both technique and scale. The paintings and drawings are characterized by the high quality of their execution and a constant desire to translate the peaceful atmosphere of the regions he visited. A large portion of his output consists of drawings and to a lesser extent paintings. He also made print versions of some of his works.