The gradient tool in MediBang Pro is located in the “Filter” tool icon, alongside color changing tools and other fun effects.

Tap to access the Gradient tool.

You can control whether or not your gradient will be linear or radial, as well as what the color scheme will look like.
Primary to Secondary will result in an entirely opaque gradient, while Primary to Transparent will give you an gradient with a gradually becoming transparent background.
Switch on Horizontal・Vertical to make the gradient snap to either a horizontal or vertical direction.