Gradient Folders are really easy to make.
  1. Drag your solid black square to make the base. 
  2. Drag again smaller where you want the tab for the folder (folder top)
  3. Choose your gradient Colors
  4. Click Icon/Insert Gradient/check mark Don't replace Mask Color and Click Apply
  5. Use the line tool to make some shadow lines and the pen to precisely put a colored pixel right where you want it.
 
Using the line tool I begin to make my shadow on my gradient filled folder.
 
I used the fill square (the black square icon on the left) to drag a square.  I clicked Icon/Insert Gradient/then chose not to replace the mask. This option fills the painted area.
 
To get your gradient colors.
On the color palette on the right ... Right Click for one color and Left click to choose the other color.
 
Remember this can also be used on text.
 
This gradient fill would make a great start for a folder icon.  Taking most of the work out of the fill and leaving you with just the shadowing to make.
 
And there you see my finished gradient folder.
 
Nice and Easy