Gennett's homer traveled 423 feet with an exit velocity of 108.5 mph, according to Statcast™.
"It's just getting the opportunity," said Gennett, who finished 2-for-3. "That's all I can ask for, and all I've been wanting for three years now. I'm glad today is the first chance I get."
Gennett said he expected all along to be in Monday's starting lineup after a strong but abbreviated spring in which he batted .429 and hit four home runs in 10 games. Two of the home runs came against left-handed pitchers.
Manager Craig Counsell has made no promises moving forward. Second base presents his best opportunity for at-bats for switch-hitting rookie Colin Walsh, and slick-fielding Yadiel Rivera also could see time there.
"I thought Scooter performed really well in the spring; it was a small sample. I think it will work," Counsell said. "I told Scooter, 'You're playing today, you're going to have to earn playing time, and that's how it's going to work.' But I thought today was right."
He added later: "He showed that he warranted being in there, that's for sure."
Gennett figures to be back in the lineup for the remaining two games of the series against Giants right-handers Johnny Cueto and Jeff Samardzija.
"I'm just looking forward to the opportunity to play baseball, and play baseball every day," Gennett said. "That's the biggest thing. Lefty, righty, it doesn't really matter. I think nowadays, it's getting so blown up on the lefty-lefty deal. There's [fewer] lefties in general, so I think there are righties that struggle against them, too, just because they don't see them all that much. Really, I think it's getting blown out of proportion a little bit. When you don't get the opportunity to face people, it's tough to hit them."
Adam McCalvy is a reporter for MLB.com. Follow him on Twitter @AdamMcCalvy, like him on Facebook and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.