MacKenzie, Alexander, History of the M'Kenzies makes no mention of a Gilbert.
page.472 has the following.
Note carefully the marriage contract vs Gilbert birthdate.
THE MACKENZIES OF MOUNTGERALD. I. COLIN MACKENZIE, first of Mountgerald, was the second surviving son of Kenneth Mackenzie, VI. of Gairloch, by his third wife, Janet, daughter of John Cuthbert of Castlehill, Inverness. He was a Lieutenant in the Scotch Fusilier Guards, and fought at the battle of Stenkirk, after which he retired from the army, purchased the estate of Mountgerald and, in 1726, built Woodlands House. He married, first, Margaret, widow of Roderick Mackenzie of Findon, daughter of Alexander Mackenzie, I. of Ballone, without issue. He married, secondly, Katharine, daughter of James Fraser of Achnagairn (marriage contract 1721), with issue - 1. James, his heir and successor. 2. Alexander, who died, without issue, in 1725. 3. Kenneth, who died in 1727, withoutissue. 4. Colin, who succeeded his brother James. 5. Isabel, who married Sir Lewis Mackenzie, VI. and third Baronet of Scatwell, with issue. 6. Anne, who married Alexander Mackenzie, II. of Lochend, with issue. Colin died in 1727, and was succeeded by his eldest son,
