is, the tangent of the angle between the line and the positive x axis.

The x axis has angle 0 with itself, so its gradient is tan(0) = 0.

The y axis makes an angle of 90 degrees with the x axis, and tan(90) = infinity, So the y axis has infinite gradient.

Your question is what is the gradient of a 10 degree slope.

If the line makes an angle of 10 degrees with the positive X axis, the gradient is tan(10 degrees) or 0.17632698 approximately.