If your windowsill doesn't have enough space to accommodate several very large plants while giving them at least 4 hours of direct sun a day (or its equivalent), you'll want to supplement; as Grey Moss mentioned T8 and T5 shoplight fixtures will do fine, just make sure it's a set that provides relatively high lumen output (if you can afford it, LED fixtures are even better because they don't amp up the heat). Temperature (and secondarily a decent relative humidity) are the next concern, as while intermediate and easy highland species do fine in most household conditions, true highlanders and lowlanders need environmental supplements to meet their needs, either by upping both temperature and humidity, or dropping nighttime temps.
The gnats you are seeing are probably fungus gnats, and Nepenthes generally don't attract or trap them in any notable manner; while they're harmless to larger plants, the larvae eat roots and will destroy small plants and seedlings.