first Simon wasn't in good form or had problems in several of those matches (on the opposite it was Gasquet's case when he lost once to Simon in Sunrise challenger).
secondly the main way to defeat Gasquet is to take him far back, and with his counter-punching style, shots with no spin, sometimes short, it's not at all Simon's style.
Often Simon plays too short against Gasquet.
Besides, a great quality of Gasquet, underrated imo (Gasquet being seen by many ones as a shot-maker, which is not his main way to play), is to be a player using a lot the spin, including the alternance with the slice, and that's the typical way to defeat Simon, Simon is great at counter-punching flat and powerful shots, but he has difficulty to control well spinned shots, especially when they don't have a lot of power and there's an alternance of spins.
If you add that Gasquet has a complete game enough to be able to sustain long rallies, you have the whole set.
To add on regarding the Gasquet-Simon matchup, look at the H2H of Gasquet-Murray. Murray leads it 4-3 but Gasquet was up 2 sets in two of the losses (2008 Wimbledon, 2010 French) and dominated early in a third loss (2012 French) - the only win in which Murray had control of the match early was 2011 Wimbledon. All three of those losses were a combination of mental and physical fatigue (clearly Murray beats Gasquet in both those areas), which means there really is a matchup issue if you only consider their styles.