You don't NEED to. If you are going to make the attic/top floor into heated/cooled living space then the insulation will be in the rafter (roof) bays with plastic vent keepers between it and the roof sheathing (plywood), rather than (or in addition to) in the floor. In this case you would need to remove or seal off the gable vents. Sometimes it is a good idea to do away with the gable vents if they are allowing too much wind into the attic. This wind can blow around the blown insulation, if you have it, or decrease the insulating values of fiberglass insulation. Fiberglass insulation only retains its full R-value with NO wind on it. It is rare that gable vents let in too much wind so you are probably fine just the way you are. The more vents the better in most cases.