A gear driven head is different from the old "impact" sprinkler heads that were installed for years. Impact heads are rarely available anymore and for good reason.
A gear driven head is a closed sprinkler head that rotates using water power within a gear mechanism enclosed in the sprinkler head. These heads have been in use for a long time now and are the only heads installed.
Gear heads are used in areas that require watering for areas between 20-40 feet.