Of the children of Gad, by their generations, after their families, by the house of their fathers, according to the number of the names, from twenty years old and upward, all that were able to go forth to war;
In simpler terms, this verse is saying: Among the descendants of the tribe of Gad, counting people based on their family groups and fathers' houses, we find those who are 20 years or older and capable of fighting in battles.