She had more than 55 percent of the vote Tuesday with more than 90 percent of precincts reporting. Democrat Dennis Campanale had less than 45 percent.
Gaines was favored in the heavily Republican 4th Assembly District, which stretches from north of Sacramento to Lake Tahoe.
Republicans account for 45 percent of the district's voters, compared to 31 percent for Democrats and 19 percent who decline to state a party preference.
Gaines' victory retains the partisan balance in the Assembly. Democrats will hold a 52-28 majority when she is sworn in.