Gerade has many different meanings, e.g. "straight" and "even", but I assume you are interested in the "at the moment" one. Actually, it's complicated. They can be interchangeable at times but sometimes they have different connotations. Ich arbeite gerade. I'm in the middle of doing some work. Possible question: was machst du gerade? Ich arbeite jetzt. That may mean the person just got a job after being unemployed before. Another possibility is that the person wants to stress that he or she has started doing some work so that the other person knows he or she is now occupied. Possible situation to trigger that answer: I changed my mind, do you still wanna go watch that new movie?