It depends what the temperatures will be. People sometimes say 'Species *** does not need heat' or something similar, but every reptile, actually, every living thing needs heat. If you live in an extremely hot place, you may not need to heat anything. If you live in an extremely cold place, you're going to have to heat every reptile you keep.
Marbled Geckoes are extremely cold tolerant - I've seen them out actively foraging at under 10 degrees celcius, but they'll try to keep themselves warmer than that and you should give them the opportunity during the day. I've never used bulbs as heat for geckoes, and if you want to use a light bulb, the wattage will depend on the size of your enclosure, the temperature of the room, etc.
Incidentally, when I live in QLD it wasn't cold enough for them and I used to put them in the fridge overnight during winter to cool them, and they'd still want to feed during the day. Basically, they're cold climate geckoes which should be given cold nights for most of the year and moderate to warm temperatures for a few hours each day for most of the year.