foo bar

Okay, the solution was more or less my own stupidity. You don’t get the variables like you might expect in $_POST, $_GET or $_REQUEST. You have to include them as a global.

For the example I started out with it would be

function xyz()
{
global $gallery;

//...
}

And also sargant was completly right – don’t forget that!

You do this because you combine two arrays, rather than two strings kategasis.

And thank you qrof..I will read this and if it helps I will post a solution.

..well okay its kind of a solution.
The simple answer: The codex is wrong in this matter. You are actually suppossed to use register_activation_hook(__FILE__,$function) on this. Whereas $function can’t be the array(&$reference, $functionname) a reference to a class-variable. The solution to still sink $this in this hook is to call it in the constructor like this:
register_activation_hook(__FILE__,array(&$this,$functionname))

Its not like it is described and like this wasn’t enough. IN 2.0 there are also errors in the register_activation_hook-function or at least in the plugin_basename-function which it uses.
So instead of register_activation_hook you will need to use
add_action("activate_{pluginname}",$function)

And in case {pluginname} isn’t a plugin which is straight in the pluginfolder this is an example of usage:
FOLDEROFYOURPLUGIN/plugin.php be aware of the normal slash.

Hmm, thank you Viper, but “I don’t know” wasn’t exactly the answer I was looking for.