You take subjects for credit points, at MQ easier subjects at 100 level are generally worth less credit points (3) than more difficult subjects at 200/300 level (usually worth about 4). You get the credit points if you pass the subject, if you fail, you do not. You must accumulate a particular amount of credit points at different difficulties at complete the degree, eg. my degree requires 96 credit points.
GPA=Grade point Average. It measures how well you are doing at uni according to the marks you are receiving, F,CP, P, C, D, HD. Someone else may want to give you a more detailed explanation of this.