I used a club hammer to loosen my gooseneck. I'm not convinced the WD40 I suggested using actually did anything worthwhile. There must be a better chemical for loosening aluminium alloy corrosion. Hopefully, someone will be able to tell you what that is.
The best way to loosen alloy corrosion is to use a 10% caustic soda solution - sparingly! Just dribble it into the joint where the square section goes into the boom fitting and leave for an hour max. You should see a bit of fizzing as the caustic starts to dissolve the aluminium oxide. Whe you have freed it off, drench it in vinegar to neutralise the caustic or it will start to dissolve the alloy. Obviously take all precautions - gloves goggles and plastic apron. I have freed off big seized winches and other alloy castings using this method.