Grams are a measure of weight.
A gram is a unit of mass. The newton is the metric unit of measure of weight. It is happenstance that on this planet (not sure about yours), gravity is constant at the surface and therefore to all intents and purposes, mass can be viewed as a weight.
You don't "weigh" 75kg, you have 75kg of mass.
Why don't you freeze your tuna? It will then be a solid instead of a liquid and they have no leg to stand on.
Or... order room service!