What should Fortress Gate's attack be?

I have a level 17 Fortress Gate, and four +Attack kingdoms at level 10. What should my Gate’s attack be?

I see the following in the Skills tab:
“+2 for Level”
“+4 for Kingdom”
“+2 for Ascension”
“±2 for Base Skill”

These do add up to the 6 attack that my Fortress Gate has, but I can’t help but wonder if it’s supposed to have an attack of 4 (before any team bonuses, of course).

I don’t know how the numbers are totalled but my gate is level 15 and has an attack of 8. And that’s just sitting in the deck, not in any team. All kingdoms level 10 but none at 5 stars.

Not sure why your Base Skill is -2 and Level is +2 (mine show 0 and 0) but your total of 6 would be correct if you’ve got 4 Attack Kingdoms and ascended your Gate to Ultra Rare.