Ground, in most electronics, is just the point in the circuit we call "Zero volts", and use as a reference when measuring voltages elsewhere in the circuit. It is where we connect the black lead of our meter.
The gate circuits on the page you mention are in a sense, only partial circuits: in use, there will be a additional circuits connected to points A and B, and to "ground", to form complete ("looped") circuits to drive the gate.
simulate this circuit – Schematic created using CircuitLab
(Ignore component values - they are the schematic editor's defaults)
The node marked "Ground" is the reference point in the circuit - it is where we put the black lead of our meter when measuring voltages elsewhere in the circuit - we must always measure voltage between two points.
With both switches open, R2 and R3 will pull Input A and Input B "High" (to +5 volts), and the output of the gate will also be High (+5 volts, as measured between Output and Ground)
If either switch is closed, the associated input will be connected to Ground (zero volts), and the diode will pull the output down to about 0.7 volts from ground (a typical forward-biassed diode will have about 0.7 volts across it)
In Real Life, the output would go on to some other circuit, and the inputs could come from other gates rather than the switches.