The reigning South American champions took a big step towards defending the trophy thanks to Rafael Santos Borre's early penalty and a well-worked Ignacio Fernandez goal in the 70th minute.
Referee Raphael Claus consulted VAR to award the spot-kick for River's opener and technology also played a part in the stoppage-time red card shown to Nicolas Capaldo, who had initially been booked for a foul on Enzo Perez.
Boca striker Ramon Abila criticised the performance of the match officials, while coach Gustavo Alfaro accused the hosts of persistent diving.
"We know the River players are always throwing themselves and seeking penalties," Alfaro said.
But his counterpart Gallardo dismissed the protests, insisting the comments were designed to deflect from the quality of River's victory.
"Whoever wants to talk about the referee is hiding from what happened," he said.
"We won because we were superior."
Capaldo wasted a glorious chance for Boca before his dismissal but River otherwise spent long periods in control as they closed in on a date with either Gremio or Flamengo in the final.
"I think we saw a winning team," Gallardo added. "We are a team that takes on the risks of being the protagonists. Sometimes we are rewarded and sometimes not.
"River demands a lot from you and we feel comfortable with the demands. The key is not to relax, because what we give the River fans is a very nice feeling. The football world sees a River that plays well."
River beat bitter rivals Boca 5-3 on aggregate in last season's two-legged final, the second instalment of which had to be rearranged and moved to Madrid following fan violence.