Before playing professionally, he graduated from St. Paul High School, where he lettered in baseball and football, and then attended the University of California, Los Angeles (1978–81, history major).

Gallego played on the United States national baseball team at the 1979 Pan American Games.

[1] Gallego was the Athletics' starting second baseman during their three-year run of AL championships from 1988 through 1990, which included a World Series sweep in 1989 against their Bay Area rivals, the San Francisco Giants.

[3] Gallego closed out his career with the Cardinals in 1996 and 1997, where he once again played under Tony La Russa, his manager while with the A's.

Following his graduation from UCLA, Niko began playing for the Visalia Rawhide of the California League.