What is 5 grams as a decimal?

A decimal number is simply a way of representing a number in such a way that the place value of each digit is ten times that of the digit to its right. A decimal representation does not require a decimal point. So the required decimal representation is 5 grams, exactly as in the question.

A decimal representation does not require nor imply a change in the measurement units. If this is required then you need to specify the units into which the quantity is to be converted.