But he then made amends when he headed the Gills' killer third on 56 minutes - his 19th goal of the season.
"That was the relief of missing the early chance. It was important to take the next one and thankfully I did," he told BBC Radio Kent.
"I'm a striker. I'm going to miss chances. Hopefully not as easy as that one in the future.
"You've just got to brush it aside and get on with it.
"It was that important goal that's going to give you that breathing space," said McDonald.
"I'm confident in front of goal and, if the boys give me balls like the one John Nutter put in, hopefully I'll get on the end of them."
Norwich are doing brilliantly and, if they get promoted, it would be brilliant for the club On-loan Gillingham striker Cody McDonald The Gills' victory lifts them up to sixth in League Two, level on points with fourth-placed Rotherham United.
McDonald, who was working as a scaffolder and playing part-time for non-league Dartford just over two years ago, is at the Priestfield on a season-long loan from Norwich City.
And he admits that he is keeping a close eye on events at his parent club, and getting excited at the thought of potentially being a Premier League player should the Canaries win promotion.
"That would be brilliant for me," he said. "I've got a year left on my contract there, so let's see what happens there.
"I keep an eye on what they're doing. They're doing brilliantly and, if they get promoted, it would be brilliant for the club."