The GPD refers to how many gallons of filtered water the reverse osmosis system can produce per day. A 50 GPD system like the APEC ROES 50 can produce up to 50 gallons of filtered water per 24 hour period. This is usually sufficient for 1-3 people. A 100 GPD system can produce up to 100 gallons per day. This additional output is better suited for medium to large households with greater water usage needs. So in summary, the higher the GPD, the more water the system can filter per day, which allows it to handle greater demand.