When you’ve settled on a scheme, you choose whether to generate a linear or radial gradient, a direction for it to render, and which browsers you want to support.

Although the default direction for a linear gradient is horizontal, you can change the angle to adjust the gradient's direction.

In Pixelmator, once you activate the Gradient tool, click the gradient preview at upper left of your document and in the resulting menu, click the black to white linear gradient icon.