Gunslinger II is Jacob's exclusive gun skill, making every Countershot a headshot. It needs level 6 and Gunslinger I.
Gunslinger II sits at position 10 of 18 in the Ecosystem tree. Unlocking Gunslinger II takes 6 skill points. You need level 6 and Gunslinger I before you can buy it. Gunslinger II is one of Jacob's three exclusive skills, so Evie cannot take it.