Tanner led the Gaels (24-9) with 24 points and 13 rebounds. He was named to the All-Tournament team along with teammate Rodney Gibson.
"Tanner's getting his confidence and he can be hard to stop, hard to match up against," Robertson said. "Rodney gets a lot of the attention and Tanner kind of sneaks up behind you."
The big second quarter gave the Gaels a 33-19 halftime edge, but the Blue Storm (17-8) cut into the deficit in the third.
Greg Kinsey hit three of his five 3-pointers in the third as Mercersburg made it an eight-point game going into the final quarter, but the Blue Storm never got closer.
Kinsey led all scorers with 27 points and was named the tournament's most Outstanding Player. Antoine McGee had eight points and seven assists and was named to the All-Tournament Team.
Gibson finished with 11 points, Matt Spierenburg grabbed seven rebounds and Josh Carter came off the bench for eight points for the Gaels, who avoided a three-game sweep by Mercersburg this season with the win.
"We picked the right one to win, the championship game of a tournament," Robertson said. "It's become a pretty good rivalry. There's a coaching friendship there (with Mercersburg's Mark Cubit). Good teams, good crowd ... it's a good high school matchup."
And as much as anything else, the three-day tournament helped Goretti keep its legs for the Baltimore Catholic League tournament, which begins Wednesday at Goucher College.
"Finding games to play was great. Finding them in this kind of atmosphere was even better," Robertson said. "I hope it kept our game sharp. That's what I was hoping for. Winning the tournament is icing."
HEIGHT="6" ALT="* "> Western Reserve beat Riverdale Baptist 70-64 in the third-place game, St. James fell to Solebury 64-62 in the fifth-place game and Bishop Walsh beat The Hill School 57-51 for seventh place.
Marcus Kinzer (Solebury) and Nick George (Bishop Walsh) were also named to the All-Tournament Team.
No. 6 Goretti 51, No. 1 Mercersburg 43
ST. MARIA GORETTI (51)
Gibson 11, Davis 2, Spierenburg 6, Tanner 24, Carter 8. Totals 19 9-17 51.
MERCERSBURG ACADEMY (43)
McGee 8, Kinsey 27, Legler 2, Campos 2, Balaban 4. Totals 16 6-8 43.
Three-point goals - Goretti 4 (Gibson, Tanner 2, Carter), Mercersburg 5 (Kinsey 5).