Unless I really forgot my algebra...if you multiply by the credit number, then divide by it...you'll just get the grade number you started with.
Try this:
Grade x Credit Hours = Quality Points (EX: A = 4 point x 4 credits = 16 quality pts)
Add up all Quality Points (will have a figure for each course you took), then divide by the Total Number of Credits. This should give you your GPA.