One gram per litre is exactly one kilogram per cubic metre. The numbers are identical — and the reason this pair has its own page, separate from the density category where the same two units also appear, is that concentration and density are different questions with the same arithmetic.
About the units
Density asks how much a material weighs per unit of its own volume: steel is 7,850 kg/m³ because that is what steel is. Mass concentration asks how much of a dissolved or dispersed substance is present per unit volume of a mixture: 5 g/l of salt in water says nothing about the density of salt and everything about the solution. The two quantities have identical dimensions and identical units, which is exactly why this site keeps them in separate categories. Combining them in one table would let the engine convert a concentration into a density without complaint, and the answer would be numerically correct and physically meaningless.
The exact factor
The factor is exactly 1, since a cubic metre is a thousand litres and a kilogram is a thousand grams. What the page is useful for is scale: g/l suits laboratory and process work, kg/m³ suits large volumes such as reservoirs, digesters and atmospheric loading. A suspended solids concentration of 3 g/l in a wastewater plant is 3 kg/m³, and the second form is the one that makes daily sludge production obvious when multiplied by a flow in cubic metres per day.
Where you meet this conversion
Water and wastewater engineering, where laboratory results arrive in milligrams or grams per litre and process calculations run in kilograms per cubic metre against flows in cubic metres per hour. Fermentation scale-up uses g/l in the laboratory and kg/m³ in the plant for the same broth. Air quality reports particulate loading in micrograms per cubic metre, which is the same family several prefixes down. Chemical dosing calculations move between the two labels routinely, and recognising that no conversion is needed prevents an unnecessary one being applied.