I can't believe the word is not in the dictionary! being of universal usage (from México to Argentina). I looked it up and the term seems to be Nahua (Mexican) in origen. Guácala is used to express disgust, loathing... Mexican use the term 'guacamole' for an icky slimy greenish sauce made with avocados. Guácala! They also have the expression 'guacarear' (or 'huacarear) for to vomit.
I have to say most mexicans don't use that as to vomit, but then again, it could depend on what state you are from because they have soooo much slang and variations from state to state.