Summon companion

Its not about tame beasts, I have one. Apparently Summon Companion is a spirit gem based skill? I input skill gems into to use similar like Cast on Crit? I found nothing on how to get it, its not in the view if I use an uncut spirit gem. Am I dumb?

Used here in the build:
https://mobalytics.gg/poe-2/builds/lightning-arrow-farmer-fubgun#how-it-works-2