Colten Brewer yielded one hit while striking out four over four innings as the Pirates blanked the Cardinals, 1-0, in their one-game semifinal.
Brewer, a 2011 fourth-round Draft pick out of Canton High School in Texas, worked around a two-out walk to Brett Wiley in the first inning and a leadoff single by Jem Argenal in the second.
The 19-year-old right-hander retired his final nine batters and the bullpen trio of Jackson Lodge (1-0), Bryton Trepagnier and Roberto Espinosa continued to put up zeros the rest of the way.
Brewer, who went 1-3 with a 3.24 ERA in eight regular-season games, was a first-team District 14 3A selection at first base in 2010. But he moved to the mound the following year, going 8-2 with a 1.43 ERA and 98 strikeouts en route to being named the district's pitcher of the year.
The Pirates shared the best record in the 14-team league but finished second in the North Division because of their head-to-head record against the Tigers. The Bucs nearly missed out on the Wild Card berth after losing their final two games of the season.
Now the Pirates will face the South Division champion Red Sox in Finals. The best-of-3 Championship Series was slated to begin Monday but was pushed back one day due to Tropical Storm Isaac.
In other Gulf Coast League action:
Red Sox 1, Tigers 0 (10 innings)
Former University of Florida closer Austin Maddox tossed three scoreless innings as the Red Sox outlasted the Tigers in the other semifinal.
Deiner Lopez drew a two-out bases-loaded walk in the top of the 10th inning to plate Nathan Minnich with the game's only run.
Tigers starter Fernando Perez, who shared the regular-season GCL lead with five wins, allowed one hit over six frames. Box score