"Be a gas" is a colloquial phrase that means to be entertaining or funny. Synonyms for this phrase could include "be a riot," "be a blast," "be a hoot," "be a laugh," or "be hilarious." All of these phrases imply a sense of enjoyment and laughter coming from a particular situation or person. It's important to note that synonyms can vary based on the context in which they are used, and what might be amusing to one person might not be to another. Nevertheless, using alternative phrases like these can add variety and depth to your language, making your conversation more engaging and interesting.