Looking at the base game XML, the Guided Missile's highest flavour is actually offense, at 15. Its other flavours are naval, defense and air at 5, 2, and 2 respectively.
I'm not sure how multiple flavours factor in here, but perhaps if your leader has both high offense and high air flavours, it might be more likely to build them. Of course, it would be more likely to build the other units associated with those flavours too, so I'm not sure you could get them to build lots of Guided Missiles in particular (especially since many military units have the offense flavour). If this leader has a UU that replaces the Guided Missile, then perhaps you can change that UU's flavours so that it's the most attractive unit to the leader.