Although we do not know the exact admission formula, Cal Poly SLO uses capped weighted GPA for grades 9-11, plus (as mentioned by @ucbalumnus) bonus points for extra coursework in some areas, and bonus points for a few other factors.
You can use the RogerHub calculator to calculate SLO GPA, but instead of grades 10-11, use grades 9-11. The SLO GPA will be the “Weighted and Capped” GPA. GPA Calculator for the University of California – RogerHub
Because of how capped GPA works, strong students with lots of coursework may actually have lower capped GPA than students who took the minimum number of courses in high school. So, Cal Poly SLO is not necessarily a slam dunk for strong students, at least in the more impacted majors.