G, as a capital can have 2 meanings: 1) as the universal gravitation constant which is a physics unit, so we exclude it in chemistry,
or it can be 2) the SI abbreviation for Giga = 10^9
When associated with a lower case m, it would normally be understood to mean a gigametre , which is a length 10^9 metres.
But I cannot see these having any relevance in chemistry.
Edit later: Unfortunately many authors use abbreviations incorrectly. Can you provide a sentence where you found this abbreviation.
Gm AbbreviationSource(s): https://shorte.im/a9wKm
It could be grams or Gigameters