One gram per litre is exactly one kilogram per cubic metre. This is the third of the metric density identities, and it is the one that matters at the dilute end — for gases, solutions and anything where the density is far below water's.
About the units
A cubic metre is a thousand litres and a kilogram is a thousand grams, so the ratio is one. What distinguishes this pair from the other identities is the range it is used in. Grams per litre is the natural unit for substances much lighter than water: air at 1.225 g/l, carbon dioxide at 1.98, helium at 0.179, and dissolved solids in water at anything from a few milligrams to a few hundred grams per litre. In those ranges g/cm³ would give figures with four leading zeros, which is why the litre-based unit takes over.
The exact factor
The factor is exactly 1. The values worth having are the gas densities at standard conditions, since that is where the unit earns its place: dry air 1.225 kg/m³ at sea level and 15 °C, water vapour 0.804, methane 0.668, and hydrogen 0.0899. Gas density depends strongly on temperature and pressure, so any figure without stated conditions is incomplete — unlike liquid and solid densities, where the variation is small enough to ignore in most work.
Where you meet this conversion
Ventilation and air handling, where mass flow is calculated from volume flow using air density in kg/m³ while laboratory measurements of the same air report g/l. Water treatment and analytical chemistry express concentrations in g/l or mg/l, and converting a concentration to a mass loading over a volume of water uses the identity directly. Gas storage, buoyancy calculations for balloons and airships, and emissions reporting all move between the two labels for the same number.