There isn't a direct Gaelic equivalent of the name "Mitzi." Gaelic names are often derived from elements with specific meanings. To find a Gaelic equivalent, you'd need to find a name with a similar meaning or sound. Since Mitzi is of German origin and its meaning is unclear, a direct translation isn't possible.
You could choose a Gaelic name that sounds similar or has a similar feeling, but it wouldn't be a true translation. This is subjective and depends on the aspect of "Mitzi" you wish to emulate (sound, perceived meaning, etc.).