"We missed our opportunity, me striking out instead of putting the ball in play," Gregorius said.
Gibson was quick to point out the duo’s successes Monday night, as they combined to drive in three runs, two on a Gregorius homer.
"You’ve got to give them credit. They both contributed," he said. "They’re younger, and it’s kind of an excitable moment and they’ll get better the more times they go through it."
It was the third walk-off win in as many days for NL-East leading Washington.
LaRoche sent a 3-1 pitch from Will Harris (0-3) off the wall in the back of the Nationals bullpen and was mobbed by his teammates at home plate.
"I got every bit of it, I did," LaRoche said after his first career walk-off homer.
Arizona tied it 4-4 when David Peralta led off the ninth with a solo homer off Tyler Clippard.
Washington trailed 3-2 when Denard Span doubled with one-out in the eighth against Brad Ziegler and Anthony Rendon followed with a triple to tie it. Jayson Werth then scored Rendon with a sacrifice fly.
Miguel Montero had three hits for Arizona, and rookie Ender Inciarte extended his hitting streak to 18 games.
Arizona had taken the lead in the top of the eighth.
Lamb drew a lead-off walk off Jordan Zimmermann, and Gregorius sent a 3-2 fastball into the Nationals bullpen for his fifth home run.
Wilson Ramos had given Washington a 2-1 lead with a two-run homer off Arizona starter Vidal Nuno in the seventh.
Nuno, winless in eight starts with the Diamondbacks, pitched well, allowing two runs and five hits over seven innings.
Zimmermann went seven-plus innings, allowing three runs on four hits.
In the fifth, the first two Diamondbacks reached base via a walk and a single. Jordan Pacheco bunt the runners over, and Lamb’s sacrifice fly made it 1-0.
Washington second baseman Asdrubal Cabrera was shaken up, but remained in the game after a third-inning collision at second base. On a sacrifice bunt attempt by Jordan Zimmermann, Cabrera slid into second and the side of his head slammed against the knee of Gregorius’s, who was fielding the throw. Cabrera remained on the ground for several minutes as manager Matt Williams and the trainer came out to check on him.
Inciarte is tied with Jose Abreu of the White Sox for the second longest hitting streak this season by a rookie. Abreu had a 21-game streak as well. . Mark Trumbo (1 for 4 with a walk) has reached base safely in 17 straight games, and hit safely in 15 of 17, going 21 for 61 (.344).
Diamondbacks: SS Chris Owings, on this disabled list with a strained left shoulder, began a rehab assignment Sunday with the team’s Arizona Rookie League affiliate, going 2 for 3.
Nationals: Werth (sore shoulder) was in the starting lineup for the first time since Aug. 10. . OF Nate McLouth, out since Aug. 2 with right shoulder inflammation, was scheduled to undergo an Arthrogram — an MRI procedure that involves dye being injected into the joint — on Monday to help determine the extent of his injury.
Diamondbacks: RH Chase Anderson (7-4, 3.16) is 2-0 with a 2.19 ERA in his last six starts.
Nationals: RH Stephen Strasburg (9-10, 2.53) pitches Tuesday at Nationals Park, where he’s 7-2 with a 2.41 ERA this season.