Both gram and AMU (atomic mass unit) are units of measure. The gram is a much larger unit than the AMU. It is a unit we can use in the everyday world--we go out shopping and ask for 500 grams of apples. The AMU is a unit used in the scientific-medical world. The shopkeeper would look at us askance if we asked for an 500 AMU of apples.
Remember that the AMU measures atomic mass. By definition, 12.0000 AMU equals the mass of 1 Carbon-12 atom. 12.0000 gram equals the mass of 1 mole of Carbon-12 atoms.
Know that a mole equals Avogadro's number--6.02_10^23. So when we talk of a mole of atoms we mean 6.02_10^23 atoms. The weight, in grams, of one mole of an element is the same as the atomic weight of that element. For example, the atomic weight of the element helium is 4. So 1 mole of helium weighs 4.00 grams.
Find the atomic weight of the element. One mole of the element equals the atomic weight of the element.
Multiply 6.02_10^23 by the number of grams in your question to get the AMU. For example, let us take one mole of helium. The atomic weight of helium is 4.00; so one mole of helium weighs 4 grams. To convert the grams to AMU we multiply 6.02_10^23 by 4 to get the answer in AMU.
The quantity of any element in grams equal to its atomic weight gives 6.02*10^23 atoms of that element.