"He's the same all the time ... he never smiles," the St. Louis manager said. "Real even keel."
Gast used that business-like approach to pick up a win in his major league debut Tuesday night.
Gast (1-0) tossed five shutout innings before giving up four runs in the sixth. He surrendered six hits, struck out three and walked one in a 71-pitch effort to help the Cardinals win for the fifth time in six games and 11th in their last 13.
"I was excited, ready to go," Gast said. "There was some excitement. I was able to refocus the adrenaline to make my pitches."
Matheny was pleased with the poise of the 24-year-old, who held the Mets scoreless on three hits and 50 pitches through the first five innings.
"He was fine on the corners," Matheny said. "You could tell he was working the bottom of the zone. Once he started going, he was making some really good pitches."
Beltran slammed a three-run homer in the fifth inning, his team-high tenth, to give St. Louis a 9-0 lead. He also added a run-scoring hit in the third.
"It was a fastball in the middle of the zone," Beltran said of the 437-foot blast that landed in Big Mac Land. "I was able to stay inside and hit it good."
The veteran outfielder was also impressed with Gast.
"It was fun to see," Beltran said. "That just shows the depth of our minor league system. They have a lot of guys like that who can come in and do the job."
New York right-hander Dillon Gee (2-5) gave up six runs, five earned, in four innings for the Mets, who lost their fifth straight despite scoring more than three runs for the first time in eight games.
Gee has allowed four earned runs or more in five of eight starts this season.
"I don't know what to say, I didn't feel all that bad tonight," Gee said. "I'm a ground ball guy and I think seven ground balls got through. I don't know what else to do."
Marlon Byrd hit a two-run homer in the sixth as New York also lost for the 11th time in its last 14 games at Busch Stadium.
Gast began the season with 32 consecutive scoreless innings for Triple-A Memphis. He was called up in part to fill the rotation spot of Jake Westbrook, who is on the 15-day disabled list with right elbow inflammation.
Jon Jay also homered for St. Louis, a solo shot in the sixth, his fourth of the year. Beltran went 3 for 5.
"It's fun, we're playing good baseball," Beltran said. "We're getting good starting pitching and the offense is doing enough to win ballgames."
St. Louis scored three times in both the first and third innings to take a 6-0 lead.
Allen Craig capped off the early uprising with a two-run single. Matt Holliday doubled in Matt Carpenter for the first run. The first four St. Louis hitters reached safely and the Cardinals scored three times on Gee's first 11 pitches.
Pete Kozma and Carpenter added run-scoring singles in the third. Carpenter reached safely four times.
The Mets' five-game skid is their longest since losing six in a row April 25-30.
"We've gone through a bad streak and it's been two weeks long," Mets manager Terry Collins said. "We can't sit here and try to make something positive out of it."
Former St. Louis pitcher/outfielder Rick Ankiel, who was signed by the Mets on Monday, grounded out as a pinch-hitter in the seventh. Ankiel received a warm ovation from the crowd.
St. Louis rookie Shelby Miller (5-2, 1.88 ERA) will face Shaun Marcum (0-3, 8.59) in the third game of the four-game set tomorrow. Miller retired 27 successive batters after giving up a leadoff hit to Eric Young in a 3-0 win over Colorado on Friday. ... New York C John Buck broke out of a 2-for-27 slump with a single in the sixth. ... Ankiel is getting paid at a pro-rated share of the major league minimum $490,000 from the Mets, which comes to $382,896, and that amount is off-set against the $775,000 he was guaranteed under the deal he was released from by the Astros. He can earn an additional $775,000 in performance bonuses from the Mets based on plate appearances.