How could I calculate my GPA? I have my GPA in the 100 point system,
how could I calculate it in 4.0 point system?
I searched, and I didn't find a good answer,
Unless it is specifically requested, do not do it. Some school even urge you not to do it. It is highly probable, that they know your scale and understand what level means you are smart and hard working. Be sure to include thing like cum laude, merit scholarships, ranking within the cohort, ... These things are easier for them to understand and help them to compare you to the other students.

Miscalculating your GPA may result in unintentional deception or undermining your actual achievements. Be careful.