A line parallel to the x-axis is a horizontal line. The equation of a horizontal line is of the form y = k, where k is the y-coordinate of any point on the line.

The equation of a horizontal line is of the form $y = k$, where $k$ is the y-coordinate of any point on the line.

The equation of the line is $\boxed{y = -1}$.
The gradient of any horizontal line is $\boxed{0}$.

5. Determine the equation of the line passing through the point $(-1, 0)$ and parallel to the y-axis.

The equation of a vertical line is of the form $x = h$, where $h$ is the x-coordinate of any point on the line.

The equation of the line is $\boxed{x = -1}$.
The gradient of any vertical line is $\boxed{\text{undefined}}$.