[2] It has only been possible to distinguish Gillis Claeissens' work from that of his father and siblings after scholars discovered a contract with the artist for the painting of a triptych.
These discoveries have allowed to recognise Gillis Claeissens as an important portraitist alongside Pieter Pourbus in 16th century Bruges.
The artist did have an illegitimate son called Gillis Claeissens the Younger who enrolled in the Guild of St. Luke of Brussels on 20 February 1601.
[3] By 1589 Gillis Claeissens had moved to Brussels where he was nominated painter in title to the governor general of the Low Countries Alexander Farnese, Duke of Parma.
[3] Until recently the corpus of Gillis Claeissens' work included no unsigned paintings and therefore his role as an artist was overshadowed by the more voluminous output of his father and brother.
Thanks to the discovery by art historian Brent Dewilde of a document published in Het Brugs Ommeland in 1983 a number of unsigned works can now be attributed to the artist.
The panels of the triptych executed in 1576 are now in the Museum of Fine Arts in Budapest, where they are still attributed to Pieter Claeissens the Elder.