Ground clearance is the gap under the gate, measured at the closest point along the whole way the gate moves. Aim for about 30 to 50 mm.
Less than 30 mm and the gate catches on leaves, dirt, and small bumps. More than 80 mm and it looks gappy, and a small child or a pet can get under it.
Two things people forget. Metal grows a little when it gets hot in the sun, so a gap that just clears in winter can rub in summer. And new paving can lift the ground by 30 to 50 mm later, which eats into the gap. Leave a little extra for both.