Uh … The imaginary closed Gaussian cylindrical  surface is made up of 3 surfaces.

呃… 假想的封闭高斯圆柱面是由3个曲面组成。

The angle for the side of the Gaussian cylindrical surface is between a  horizontal electric field and the direction of any dA on the side which is always 90 degrees.

高斯圆柱面侧面的夹角是水平电场与侧面任意 dA 方向的夹角，始终为 90 度。