How many teaspoons are in a gramme depends on how dense the substance is. One teaspoon of sugar is about four grammes, while one teaspoon of salt is about six grammes. One gramme of water is equal to 0.2 teaspoons, so 5 grammes of water is equal to 1 teaspoon.
Grams and teaspoons are different units of measurement. Grams show how much something weighs. Teaspoons are used to measure volume, or how much space an ingredient takes up. Some ingredients, like salt and sugar, may have the same amount by volume, but their mass and density are very different. Using a general conversion with different ingredients, especially on a larger scale, can ruin a recipe because of the differences in mass.