I think it would be more accurate to explain it in pictures than counting numbers.
1. In Photoshop
If you look at the Photoshop screen, you can see the center axis of the gradient and the line, and the direction of the line is the starting point.
2. In CSS
You can think of the color starting direction as being drawn counterclockwise. In Photoshop, 70 degrees Celsius is -70 degrees Celsius in css.
0 degrees in Photoshop is 90 degrees in css.
It's drawn from left to right.
In Photoshop, -20 degrees is 110 degrees from css to 90 degrees + 20 degrees.
It is added up because the axis rotates counterclockwise.