Is it safe to take gravol and melatonin together?

No, it is not really recommended. Since gravol has a sedative chemical in it, it will make you want to sleep. With melatonin, although it is a receptor to effect your cranial cavity (brain) with a hormone that it's time to sleep. by taking both you would be overly tired, resulting in prolonged drowsiness, feeling of drunkenness, ECT. If you have stomach ache go with gravol, if you just wanna sleep use the melatonin.