https://en.wikipedia.org/wiki/Orders_of_magnitude_(mass)
The table at right is based on the kilogram (kg), the base unit of mass in the International System of Units (SI). The kilogram is the only standard unit to include an SI prefix (kilo-) as part of its name. The gram (10â3 kg) is an SI derived unit of mass. However, the names of all SI mass units are based on gram, rather than on kilogram; thus 103 kg is a megagram (106 g), not a kilokilogram.
The tonne (t) is an SI-compatible unit of mass equal to a megagram (Mg), or 10^3 kg. The unit is in common use for masses above about 10^3 kg and is often used with SI prefixes. For example, a gigagram (Gg) or 10^9 g is 10^3 tonnes, commonly called a kilotonne.
One context where I have seen this used is carbon stocks, e.g. petagram of carbon (PgC):
https://www.pmel.noaa.gov/co2/story/Carbon+Cycle
Of course Gigatonne of Co2 is also found very frequently.