It is defined as the total amount of heat produced, when unit mass/volume of the fuel has been burnt completely and the products have been cooled to room temperature (~ 15 0C).
When the fuel, containing hydrogen, is burnt, the hydrogen undergoes combustion and converted into steam. When the products are condensed to room temperature steam is converted into liquid water and latent heat is evolved. This latent heat of condensation of steam so liberated is included in the measured heat and so the value is high. Therefore, gross calorific value is also known as higher calorific value.