TYPO3 Core

As far as I can tell from checking the latest TSref, this particular aspect has not changed since then.

Which GIFBUILDER property would that affect?

I think this would need to be set on the respective IMAGE objects.

OK, and which properties of the IMAGE object? Or: How do you set blend-mode "over"? Concerning the structure of TypoScript properties I think the other blend modes should work similarly then?!

The blend-mode "over" is currently hard-coded in class.t3lib_stdgraphic.php (at least in TYPO3 4.7, don't know about 6.x). See the function combineExec(). What we need is a new parameter "blendMode" for the IMAGE object that is then used to generate a suitable GraphicsMagick command.

The file is GraphicalFunctions.php in 6.x.

However, GraphicalFunctions.php is only the place, where it is hardcoded one time, but when you need the option for the IMAGE object, then you should edit the PHP file, which defines the IMAGE object, add a new option there and remove the hardcoded parameter from combineExec() (as otherwise you would get a command with two blend modes).