The gaussian tool allows for the terrain to my modeled after the 3D gaussian mathematical function,

A*exp(-u(x,y)2/2*sigma2), where:

A is a height constant

Sigma alters the shape of the curve

 

To use a tile must be selected, and the enable button must be checked.  Note the standard spinner tool "M" means manual edit the number

 

Note: to speed up preview mode, the changes are not finalized until the tile is unselected.  This makes a dramatic difference for computation time however for very large terrains this will a few seconds after finalization and the status of the change (percent) will be displayed in the status area

 

Note: If there isn't any terrain to click on goto the menu TerrainOptions->GenerateTerrain