Why would you want to place additional gates? When applying lean and agile methods, you typically want to remove, or at least reduce the burden of, gates in the life cycle.
At one time, Construx had a white paper called "Introducing Agility into a Phase Gate Process". It was a well-reasoned look at how agility and gates could co-exist, with a couple of different models. However, it doesn't appear to be on their website anymore, so I'm having difficultly finding it. Perhaps you or someone else would have better luck.