Hi John2525,
There's some 'bias' to the question that you're asking, so I'll do my best to keep my answer analytical. These days, a 620 is right about the 70th percentile, meaning that about 70% of Test Takers can't score that high. Whether that is an 'easy' task for you or not really depends on how strong of a Test Taker you are and how effectively you can train to score at that level (or above).
Many Test Takers spend 3 months (or more) of consistent study time before they hit their 'peak' scores - while you might need more or less time than that (as your 'peak' might be higher or lower than 620), the task will require some commitment (and consistency) on your part. By comparison, it's not clear how long it would take you to earn the remaining 15 credit hours that you would need.
1) Have you done any study (or research) into the GMAT at all?
2) What type of deadline are you on?
It might actually be in your best interest to study for the GMAT for a short period of time, then take a FULL-LENGTH practice CAT Test, so that you can get a sense of your strengths and weaknesses and how much of a challenge it might be for you to train for the GMAT.
GMAT assassins aren't born, they're made,
Rich