The phrase "firm as Gibraltar" is commonly used to express unwavering strength, steadiness, and reliability. However, there are many other synonyms that can be used to convey the same meaning. For instance, unshakable, resolute, and steadfast are words that can be used to describe someone or something that is strong and unwavering. Similarly, immovable, unyielding, and inflexible convey a sense of solidness and stability. Other possible synonyms for "firm as Gibraltar" include uncompromising, determined, stubborn, and obstinate. Ultimately, the choice of synonym depends on the context and tone of the sentence, but these words can help to add precision and nuance to one's writing or speech.