It is a unit of mass.
Yes, 0.5 grams is equal to half a gram. The term "half" indicates that it is 50% of a whole unit, and since 1 gram is the whole unit, 0.5 grams represents half of that.
milli- represents thousandths so 2 of them is one-five-hundredth.
One gram is equal to one gram.
It is "gram", which may be abbreviated to "g".
One gram equals 1000 milligrams.