[Gimp-user] Gradient tool : how to modify the gradient ?

[PierUgo <pehache 7 gmail com>]

Hi,

I'm trying to use the gradient tool, but I would like to modify the 
gradient law. I guess that the modification takes place here :
http://prntscr.com/dkv3rp
but I can't do any modification.

When hovering the black and white triangle it says "click : select; drag 
: move.." etc, but these triangles don't react to any mouse action.

I have the same issue either on macOS or on Windows, so I probably miss 
something in the Gimp usage itself.

Gimp 2.18

Any help appreciated