[TYPO3-english] gray effect in GIFBUILDER not working

[Bing van Moorsel | Idar Creative]

Dear list,

im trying to make some grayscale images.
now i've stripped down allot of my typoscript for testing purpose:

page.10 = IMAGE
page.10.file = GIFBUILDER
page.10.file {
   10 = IMAGE
   10 {
     file = fileadmin/templates/images/button-bg.png
   }

   20 = EFFECT
   20.value = gray | gamma=0.5 | flip
}

Now i get my image button-bg.png upside down with a gamma of 0.5 but not 
gray! no matter what effect i fill in they are all working but the gray 
effect just doesnt do a thing.

what can i check/add to fix this ?

Thank you for you time.

Bing van Moorsel