If anyone was descended from the Earls of Cassilis they would be called Kennedy, not Cassilis. Here is a link to the entry for the Kennedys of Cassilis in the Scots Peerage. It is open at the page that deals with John and Hew.
https://archive.org/details/scotspeeragefoun02paul/page/474/mode/2up
All that one says of Hew's son Gilbert is that not much is known about him??
This reference suggests there was a change of name??
https://deriv.nls.uk/dcn23/9536/95363494.23.pdfGilbert [senior - fourth Earl of Cassillis??].... married Margaret, only daughter of John, ninth Lord
Glammis. By this lady, who afterwards married John, first Marquis
of Hamilton, the Earl had three sons. This Earl went by the
name of 'the King of Carrick.' He died in December 1576, and
was succeeded by John, fifth Earl of Cassillis, eldest son
and heir. [Not Gilbert's brother John - he was 6th Earl of Cassilis. Not sure how Hew fits in here???]
In a curious MS. in the British Museum, containing an account
of the Scots nobility in the reign of James VI., this is stated : ' The
ERLE OF CASSILIS, called Kanethy, being with his friends of the
same surname upon the west seas, in the Countie of Carrick, a
stewardrie and parcel of the shiredome of Ayre. There is of the
same name, in that Countie, and descended of his house, sundrie
lordis and gentlemen, whereof the principall is the Lord Barganye
and Blairquhoy, of little less living than the Erie himself. His
chief houses, Be CASSELLS and Dunnyre, 4 miles from the
bridge of Doune. The people are mingled in speeches of Irish
and English, not far distant from Carrickfergus in Ireland. This
mode of spelling Kennedy (Kanethy), seems favourable to the
supposition that the name was originally Kenneth.'
The founder of the Cassels branch, being probably driven
from Ayrshire in some of the feuds in which his family so often io 'ullte Jamili) of (Eitssds.
bore a leading part, settled in Linlithgowshire or counties adjacent,
and changed his name to CASSILLIS, the name of his ancestral seat
in Ayrshire. This has been corrupted in its spelling to CASSILLS,
CASSILIS, CASSILS, CaSSELLS, and to CASSELS, as it is now generally
spelt. The descent cannot now, however, be traced with certainty
much further back than the parish registers extend, which are
many of them in a very mutilated and defective state.
In old charters and State documents signed by the early Lords
CASSILLIS, the name is frequently spelt Cassles, CASSILS, and
Cassills. For many generations the family had used the same
arms and crest as those borne by the Marquis of Ailsa ; and in
1864, WALTER GlBSON CASSELS, of Blackford House, Edinburgh,
thought it desirable to strengthen his right by petitioning the Lord
Lyon King of Arms for permission to continue to use these arms,
with such difference and as ' nearly approaching to the aforesaid
insignia as accordant with the Laws of Arms.'
