Very. The Gel sits behind the lens, and is easily affixed to it with a couple of pieces of sticky tape wrapped around the tabs that protrude near the top. The tape isn't strictly necessary, but does have 2 benefits - firstly the gel is kept taut against the lens, eliminating optical errors that might otherwise arise, and secondly, it would otherwise be a PITA to keep the gel aligned with the lens whilst you put the fascia screws in.