At 9th level, the Gloomblade archetype allows the following shadow effect, found under Shadow Weapon training; "During the gloomblade’s turn, he can increase his reach with that weapon by 5 feet." https://www.d20pfsrd.com/classes/core-classes/fighter/archetypes/paizo-figh ter-archetypes/gloomblade-fighter-archetype/
Does this merely mean that a gloomblade's reach increases for the sake of attacks during their turn, as per the Lunge feat? Or does this ability allow the gloom blade to effectively increase the extent of their threatened squares, using this ability to perform attacks of opportunity at 5 feet more than they would otherwise?
The full of the ability makes a difference.
At 9th level, whenever a gloomblade creates one or more shadow weapons, he can apply any one of the following shadow effects (chosen upon creation) to one of them.
During the gloomblade’s turn, he can increase his reach with that weapon by 5 feet.
The gloomblade can increase a thrown shadow weapon’s range increment (if any) by 20 feet.
When he hits a foe with a melee attack with the weapon, he can use it to perform a reposition combat maneuver against the creature hit as a swift action. This does not provoke attacks of opportunity.
Attacks made with the weapon ignore hardness equal to or less than the gloomblade’s fighter level.
For every 4 levels he has beyond 9th, a gloomblade can apply an additional effect to his shadow weapons, to a maximum of three effects at 17th level; these effects can be split between multiple shadow weapons as a gloomblade desires, but they don’t stack with themselves.
The reach one is the only ability that specifies "during their turn", and the effect is given to the weapon when it is created. Thusly, the reach only applies during the character's turn, and would not increase their threat range (unless somehow an opponent provoked an AoO during the Gloomblade's turn).
"During the gloomblade's turn" seems pretty self explanatory to me. I don't see any way you could interpret that to mean during times when it isn't the gloomblade's turn.
@Dave Justus if you isolate that part as in the first post, it can easily be interpreted as something you can only activate on your own turn but then continues to function. that's not the case obviously but it is a valid interpretation of that section of the text in isolation.