To say that all German nouns ending with “anz” are feminine would be inaccurate, but there is a grain of truth in that. All German nouns that have the suffix -anz are feminine, as in:
die Dominanz (dominance)
die Intoleranz (intolerance)
die Kulanz (goodwill, fairness)
die Resonanz (resonance)
There are about 400 such nouns. As you can see, they usually correspond to English nouns with the suffix -ance (because they both originally come from Latin words with the suffix -antia).
There are a small number of (rather uncommon) masculine nouns that end with “anz” that is not a suffix:
der Mummenschanz (a dated word for mummery, a performance in masks)
der Popanz (a person of overestimated dangerousness; may also refer to a person who appears to be powerful but is in fact controlled by others, i.e. a “puppet”)
All monosyllabic words that end with “anz” are masculine as well. Those are the following:
der Kranz (wreath, garland)
der Schwanz (tail)
der Tanz (dance)
Compound nouns derived from these, such as der Blumenkranz (floral wreath) or Schweineschwanz (pigtail), are all masculine as well. There are about 400 such compound nouns; that is, the number of masculine nouns ending with “anz” is about the same as the number of feminine nouns with the suffix -anz, but you will be able to recognize the masculine ones easily if you memorize the list above.
Furthermore, two names of cities end with “anz”: Byzanz (an older name for Constantinople) and Konstanz (a city in Southern Germany). Both nouns are neuter, but since proper nouns are mostly used without articles, you will never see “das Konstanz”, but the article may appear when combined with adjectives, as in “das schöne Konstanz” (the beautiful Konstanz). Note: Don’t confuse the name of Konstanz with “die Konstanz”, which means “constancy, permanence”.