Gunslingers aren't a competitive class, and shouldn't really be chosen as your first ever character.
They're expensive, their gears are limited and weak, and their skills are severely underpowered in Renewal (they were actually quite good in Pre-Re!)
As they have a level cap of 99, they're unable to make use of the newer leveling zones and content, so they'll always be limited in what they can do until Rebellion, which has no set date for implementation here. I repeat, there's no point in waiting for Rebellion, as it has no immediate plans for iRO implementation and may not arrive here for years.
Slinger job bonus stats are pretty nice, especially the 9 DEX and 7 LUK, but it doesn't cover their weaknesses. Renewal is gear-oriented and Slingers are always going to be weaker than other classes, as they can't trans or breach 99. They're an artifact of Pre-Re that kRO has ignored up until recently.
And about the skills, Gatling Fever, which is the greyed-out skill that you circled requires Trigger Happy Shot Lv7 and Desperado Lv5, neither of which you have. You need those before it'll become available.
As Slingers have a great number of skills designed for varying builds, you can never get everything. You'll just have to choose whichever ones you like the most. As a perma-99 class, you'll always have free and unlimited access to skill resets, so make use of them to find out which skills you prefer.