In the tooltips and the Factoriopedia, Gun Turrets list a range of 18 tiles and Behemoth Spitters list a range of 16 tiles. So one would think that Gun Turrets have 2 tiles of leeway before a Spitter walks close enough to shoot back.
But this is not the case, the actual leeway appears to be 1 tile. It looks like Spitters target the edge of a hostile turret, while Gun Turrets target the center of a hostile unit, which creates a discrepancy in how the "range" property works for one as opposed to the other.
Why does this matter? Well I was trying to build defenses that could kill Spitters in that 2 tiles of leeway, and only after many failed attempts did I finally discover it's actually only 1 tile of leeway. The game was lying to me! Sort-of.