This is solved now. :3 Apparently you have to add ".value" and use parenthesis. I guess it's a JS thing. :B In case someone else tries this (you can also use "loseItem"):
$gameParty.gainItem($dataItems[$gameVariables.value(6)], -1);
The same goes for the conditional variables I tried to use:
$gameVariables.value(var) == 1 && $gameVariables.value(var) == 3;
Many thanks to boe and an RPGMakerWeb user by the name of Tommy Gun. ^_^