The phrase "gee whiz" is a sort of exclamation that means something like "wow" or "golly." There are many synonyms for this phrase that can be used depending on the context and the tone that you want to convey. For example, you might say "holy cow" or "holy mackerel" to express surprise or amazement. Other synonyms include "goodness gracious," "oh my goodness," and "good grief." Some people might also use more informal and casual phrases like "wowza" or "hot dang" to express excitement or enthusiasm. Ultimately, the best synonym for "gee whiz" will depend on the situation and the speaker's personal style and preferences.