This paper introduces G-Impact, an agent-based model that combines modelling of household consumption and belief diffusion. Household decisions integrate personal impacts (quality, cost), perceived consequences (climate change, human responsibility), and social norms. The evaluation of these different criteria relies on household beliefs, which can be exchanged during social interactions. These beliefs can be used to explain household decisions on a macro and micro scale, and thus to target information or incentive policies. The model is applied to dietary choice in France, among the omnivorous diet (INCA3), the flexitarian diet and the vegetarian diet. The costs and greenhouse gases emissions of the different diets are initialized from real data. The initial beliefs of households are derived from opinion surveys. In the control simulation, we observe a significant increase in the proportion of flexitarians, and a slight increase in the proportion of vegetarians. We also illustrate the need to properly inform households with the emergence of fake news.