Mets' Robert Gsellman to pitch in front of his grandmother for the first time Tuesday in Los Angeles

LOS ANGELES — Robert Gsellman’s first full season in the big leagues has been shaped by peaks and valleys. But good or bad, no matter what, the Mets righthander can count on reaching for his phone and seeing a sliver of encouragement.

The text message comes from his grandmother, Marilyn, who has never watched him pitch in a professional game. That will change tonight, when Gsellman faces the Dodgers, not far from where he was a standout baseball and basketball athlete at L.A.’s Westchester High School.

“It’s kind of cool being back home but it’s also kind of weird at the same time,” Gsellman said. “But I’m glad to be back home. My grandma can see me for the first time playing. So that will be awesome.”

Gsellman last pitched at Dodger Stadium in 2009. As a sophomore, he tossed a shutout to lead Westchester to the Division II Los Angeles city championship.

“I pitched here in high school,” said Gsellman, who was later chosen by the Mets in the 13th round of the 2011 draft. “But that doesn’t count.”

However, his start tonight will count for the Mets, who need him to help stabilize a starting rotation that remains at the center of any hopes of getting back into postseason contention.

With Matt Harvey on the 10-day disabled list with a stress injury to his right shoulder, the Mets still need a starter to oppose Dodgers lefty Rich Hill tomorrow. Before last night’s series opener, manager Terry Collins said righthanders Tyler Pill and Rafael Montero were the primary contenders.

But Montero pitched 3 2⁄3 innings of long relief Monday night, summoned after Zack Wheeler was roughed up for seven runs in two innings.

Montero, 26, is 0-4 with a 6.49 ERA in three stints with the Mets this season, and before the game, Collins said: “Maybe it’s the right time. Maybe, his confidence is up to where he trusts himself. See how he does.”

But Montero threw 64 pitches Monday night and that means Wednesday’s assignment will likely fall to the soft-tossing Pill, 27, who has posted a 3.75 ERA over 12 innings for the Mets this season, including a pair of spot starts.

The uncertainty surrounding what the Mets might get tomorrow only underscores how much they’ll need a bounceback from Gsellman tonight. Thus far, it has been a season of extremes.

When Gsellman’s ERA rose to 7.07 earlier this season, he seemed to turn a corner after he was sent to the bullpen, where he turned in a pair of encouraging relief appearances.

Back in the rotation by May 24, Gsellman began a four-start stretch in which he went 3-0 with a 2.16 ERA. Suddenly, he resembled the second-half savior he was last season, when he emerged to help carry an injury-riddled rotation.

But Thursday brought a step backward. The Nationals pounded Gsellman for seven runs in five innings. He allowed a career-high 11 hits on his way to snapping his personal three-game winning streak, the longest of his career.

“That’s never fun. It’s a little embarrassing,” said Gsellman, who is 5-4 with a 5.50 ERA. “That happens. That’s life, you just keep pushing.”

Often times, that extra push comes in the form of a message from a grandmother who never forgets to send a word of encouragement.

“She texts me after every start,” Gsellman said. “That’s one text that I always look forward to, good or bad. She always puts a smile on my face. Every single start.”