In a game lacking in chances, a signature Ronaldo free-kick from 30-yards gave the La Liga champions the lead eight minutes into the second half.
Luka Modric came within inches of doubling the score for the Champions League holders in the 65th minute as his low 25-yard drive was tipped on to the post by Marcelo Grohe.
The win sees Madrid become the first side to win back-to-back CWC titles and ensure the trophy heads back to Spain for a fourth consecutive year.
Having deployed some of the best of their attacking talent in front of the crowd in Abu Dhabi, Madrid dominated proceedings from the start but Gremio’s defence proved challenging to break down.
Karim Benzema saw his shot blocked for a corner before Modric’s whipped cross was cleared to Dani Carvajal, whose fierce strike was deflected behind in the best move of the opening 20 minutes.
Gremio began to grow into the match and after a poor tackle from Casemiro, Edilson lined up a 35-yard set piece, but his strike just sailed over Keylor Navas’ bar.
After a frustrating 40 minutes for Ballon d’Or winner Ronaldo, the forward was fouled 20 yards out but his swerving free-kick sailed high over the bar as the sides went into the break level.
However, Ronaldo scored the winner as he brilliantly curled the ball around Gremio’s wall and into the bottom left-hand corner after the interval for his seventh goal in as many matches in the tournament
Madrid continued to dominate after opening the scoring and ‘keeper Grohe pulled off an impressive save to stop Modric doubling their lead.
For all their effort, Gremio struggled to maintain any meaningful hold on the ball and Madrid cruised to a simple victory.