The word "gripe" can be defined as a mild or petty complaint or criticism. However, there are various synonyms that can be used interchangeably, depending on the context. For example, "whine" and "complain" can be used to describe someone who is constantly expressing dissatisfaction. "Murmur" and "mutter" can refer to expressing low or indistinct complaints. "Grumble" and "moan" can imply a more vocal and audible discontent. "Critique" and "disapprove" can be used when providing more constructive feedback on a certain issue. Ultimately, the choice of synonym would depend on the tone, level of criticism and context in which it is being used.