The gunner class have a unique +3 combat bonus when attacking units in melee range, however this bonus is not mentioned anywhere. My suggestion is to improve the gunner tooltip to mention this bonus, the blue colored text is addition I think should be added to the gunner tooltip:

GUNNER

 Ranged-combat Unit. Fights at full  Combat Strength even when defending against close-combat attacks. Increased combat strength when attacking at close range.

This is a pretty big oversight on the tooltip!

Here is the tooltip for heavy cavalry, link to chariot which is a heavy cavalry unit.

HEAVY CAVALRY

Fast, hard-hitting mounted Unit able to charge enemies. Receives bonus  Combat Strength if attacking non-adjacent enemies. Unable to climb Fortifications. 

Here the charge bonus is mentioned: "Receives bonus  Combat Strength if attacking non-adjacent enemies."

So no reason why the gunners "Focus Fire" bonus should not be mentioned in their class tooltip.

I just discovered it and completely agree with OP. Except that it does not apply to all the units with gunner class. Like, Dragoons do not get it, but Riffles and Partisans do. Even HK Wiki does not mention it. I will try to edit it, but we need to investigate further to see which units end up getting focused fire trait.

Tigranes wrote:

I just discovered it and completely agree with OP. Except that it does not apply to all the units with gunner class. Like, Dragoons do not get it, but Riffles and Partisans do. Even HK Wiki does not mention it. I will try to edit it, but we need to investigate further to see which units end up getting focused fire trait.

Good finds