The Spellslinger’s primary attributes are Support Power, Insight, Moxie, and Grit. Insight gives us more Support Power, Moxie gives us Crit, and Grit gives health. You should prioritize Support Power gains above most everything. Your largest SP gains will come from your weapon, then stacking insight as much as possible.
Secondary attributes of importance include Critical Hit Chance, Critical Hit Severity, Focus Recovery, Focus Cost Reduction, and to some extent CD Reduction and Shield Recovery rating. If you are having focus issues (probably because you do not have gear with “specials”) then you should aim to increase your Focus Recovery rate. Otherwise, focus on Crit ratings.
These values are base, without rune specials factored in. Once you are 50, getting to 1650 SP and at least 20% Critical Hit Chance should be your first goal, while still being able to manage your focus.