hi,
2 possibilities:
one way:
create a solid layer, or use the graphic layer on which you want the effect.
with the mask tool, draw a rectangle that falls mainly outside the solid, but overlaps where you want the gradient to be.
like this:
+---------------------+
| |
+-------------+--+ |
| | | rectangular |
| solid | | mask |
| | | |
+-------------+--+ |
| |
+---------------------+
Now give that mask a mode of subtract and a horizontal feather until it looks right. You can move the mask to the left or right to fine-tune, or use the mask-expansion parameter
other way:
add a transition -> linear wipe-effect
settings:
Transition Completion: 20%
Wipe Angle: -90