"It was good to get in there - one and done - so I can get back to the village, put my feet up and prepare for one hell of show tonight.
"You can burn a lot of energy being nervous. You can think about it every now and again, but I try not to focus on it because come tonight you'll be flat as a pancake."
Gill threw within 3cm of his personal best. He heaved 20.80m with his third and final attempt, despite battling a broken right foot.
He says it's a dream come true.
"We've been throwing against each other since we were about 13 years old and it's so cool to make an Olympic final together [with Walsh].
"I was a bit sore on the second throw but didn't feel it in my third because I was so pumped."
American Ryan Crouser threw the best qualifying attempt with 21.59m.
Walsh set a personal best of 21.78m when he won the world indoor championships in March.
Two-time defending Olympic champion Tomasz Majewski will be in the final 12; the Pole threw 20.56m. American Joe Kovacs, who threw 21.93m to win last year's world championships, qualified, as did Commonwealth Games champion O'Dayne Richards.
Another former world champion, David Storl of Germany, also made it through.