There are several ways to design a gradient-on-a-path that will animate when used as a Gradient Wipe transition source in Premiere or After Effects. One way is to slice your picture into pieces, and use the Gradient Tool with progressively lighter (or darker) settings to build an 8-bit gradient. Another way to make a gradient on a path is to stroke a path. Paths can be created in Photoshop with the Pen Tool, imported or pasted from Illustrator, or converted from a selection.

Here's an example where the Gradient Tool was applied to each of 5 pieces of 2 letters. The Brightness values (of HSB in the Color Picker) for Foreground and Background colors use were 0/19, 20/39, 40/59, 60/79, and 80/99. You'll probably want to reserve pure black and white to have some control over the beginning or ending of the animation in Premiere; the Background layer has 100% brightness. The source to be animated is below on the left; on the right is the gradient used for the reveal the source.

The animation at the top of the page is made in Premiere by transitioning from black to the source using the gradient. If the animation is to be composited, then transition is from the bottom layer to the animated source - but followed by a keyed/alpha instance of the source in the Superimpose track. Above is the Construction window for the animations; the softness is reduced to let the animation finish before the white background appears.

Paths can also be created in Photoshop with the Pen Tool, imported or pasted from Illustrator, or converted from a selection. To convert a selection to a path, make a selection, click the right arrow on the Path palette, and select Make Work Path.

To stroke a path, select the path, click the right arrow on Path palette, and select Stroke Path. The stroke method is determined by the Options for the chosen Paint Tool (double-click on the Tool palette to access options). To make the stroke a gradient, set Fade to an appropriate gradient luminance range or number of steps between the Foreground and Background colors. Multiple stroke paths can be designed within the same picture, but requires careful planning since the gradient steps effect the smoothness and speed of the animation. Below is an example of a stroked star. Brush Options and custom brushes can be adjusted to produce interesting results.