He joined the company in 1993. If I recall correctly, he had worked at one of his father's mining companies before that. He was appointed as COO in 2003 and replaced his father as CEO in 2009. I can't find details of his father's % of ownership - it is not detailed in the annual reports as far as I can see. Again, if my memory serves me correctly, his father had a controlling interest right up the point where Denis O'Brien started sniffing around in 2009.
But the issue isn't really about Gavin - the issue is that what happens at Independent Newspapers is remarkably similar to what happens at BOI in terms of golden circles and crony capitalism. But once again, Shane Ross's criticisms are remarkably blinkered and one-sided.