Electricity, gas and internet are all additional costs, as these are usually not included in the rent. One exception might be if you’re in student housing, where these are often all-inclusive, to make things easier for you.
Usually, utilities can vary from €50-€100+/month depending on whether you live alone or in shared accommodation. If this is the case, the more people you live with, the higher the energy use will be, but you’ll still spend a lot less than you would if you were living on your own.
Internet can cost anything from €30-40, or over €100 if you get superfast internet and TV. Again, things are cheaper if there are multiple people in the house. Be careful to read the small print! Providers in Barcelona are well-known for their exceptional deals (on the surface) which actually involve extra installation charges, or which triple in price after the first month. If you don’t know Spanish very well, have someone else check it over for you!