DISCLAIMER: This is NOT to steal content or builds, I just want the best of the best for my stat stick on my (top tier) [Gara Prime] Build. All sources are linked below.

When modding for slash like this, by default (can be changed in settings, which I recommend.) you need to HOLD ability 1 ([Gara]) to use the slash damage type (if using the other attack, its puncture, and these mods wont benefit)

If no Riven, I would suggest using another weapon like Skana or Mire, or simply put on more combo duration or puncture/slash/elemental

Combo count DOES make a difference.

Crit does NOT make a difference.

When modding a stat stick for Gara, the importance for mods goes as follows (from best to worst): Damage <- Slash/Puncture/Elemental Damage <- Combo Duration. Reddit full explanation + more stat sticks

I got this build from Leyzar (except Riven), and its modded from best to worst. Leyzars video

The Riven is also godroll, no stat could make it better (but damage should be highest stat), harmless negative for buffing the other stats, if you want to use the melee, which is not recommended, the negative is not harmless = not godroll, but there is harmless ones, like "-finisher damage" or something.

Enjoy!