Documents filed to the stock market show Mr Gilbert, also chief executive at Aberdeen Asset Management, sold 476,037 shares in FirstGroup at a price of 117.25 pence per share.
That would have netted Mr Gilbert, who has been on the board of the Aberdeen company for 27 years, in excess of £580,000.
Earlier this year he announced he was stepping down as chairman after FirstGroup staged a £515m fundraising to tackle its debt.
Since then he has been steadily selling off his holdings in the business.
FirstGroup confirmed there is no set date for Mr Gilbert to leave but a search for his successor is ongoing.