Blues defenseman Scott Perunovich turned the puck over inside the offensive zone blue line trying to reverse the puck to Pavel Buchnevich and was called for slashing as Terry was going in on a breakaway.
"He's got to be a little bit more cautious and understand the situation a little bit better," Blues coach Craig Berube said. "He's young (23), he's learning on the fly in the NHL, and sometimes there's going to be mistakes."
Buddy Robinson scored, and Anthony Stolarz made 23 saves for the Ducks (16-9-5), who concluded a five-game road trip 3-1-1.
"Obviously, when you have a road trip like this, you kind of come together as a team and we're traveling around going city to city," Stolarz said. "Guys are hanging out in the hotel and whatnot, and that kind of brings that camaraderie together."
Nathan Walker had a goal and an assist, and Gillies made 36 saves in his first NHL start in almost three years for the Blues (15-8-5), who had their three-game winning streak and seven-game home streak end.
Gillies signed a one-year contract Dec. 8 with goalie Jordan Binnington in NHL COVID-19 protocol and Ville Husso out with a lower-body injury. He last played in the NHL with the Calgary Flames against the Vegas Golden Knights on April 7, 2018, when he made 26 saves in a 7-1 win.
"It's nice to play a hockey game," Gillies said. "Obviously, the gap between the starts in this league in particular was honestly much bigger than I thought it was going to be, but the belief never wavered. I know that I can play in this league, I know that I can be very successful in this league. From a personal standpoint, it was just kind of nice to be back is the best way to put it."
Robinson scored his first of the season on a rebound in front to give Anaheim a 1-0 lead at 12:32 of the first period.
Oskar Sundqvist's first of the season, a one-timer off a pass from Walker, tied it 1-1 at 6:43 of the second period.