The Clonliffe man was drawn in lane three after running the fastest time in qualifying but was edged off the podium in a blanket finish that saw the top five positions separated by just three tenths of a second.
Russia's Vladimir Krasnov took gold in 45:59, beating Brazil's Anderson Henriques (45.50) and Jamaica's Nicholas Maitland took bronze in 45.63, just two tenths of a second faster than Gregan (45.83).
Ferrybank AC's Jessie Barr (UL) finished sixth in the 400m hurdle final in 57.65 behind Ukrainian Anna Titimets, who won in 54.64.
Haye and Fury fight on
BOXING: David Haye and Tyson Fury look set for a heavyweight showdown in Manchester on September 28 after both boxers' camps confirmed the deal.
Haye and Fury will officially confirm details of the fight tomorrow.
The announcement comes one day after Haye posted a video on social media sites purporting to show him signing a contract for the hotly anticipated contest.
Fury did not comment on Haye's claims but the fact his promoters Hennessy Sports added their name to the press release suggests the fight is now a done deal.
Ireland driving for title
GOLF: Ireland are in a strong position to claim a top-flight spot at the European Amateur Team Championships at Silkeborg in Denmark after a solid opening-day.
Bidding to regain the title they won in 2008, Ireland ended the day tied in third place with Scotland knowing another reasonable display today will take them into the last eight for the match play phase. With five of their six cards counting, they finished on 11-over-par 371, 10 shots behind leaders France.
O'Brien sees off Walden
SNOOKER: Three matches yesterday were decided on the final frame in the opening round of the Australian Open, with Fergal O'Brien, Dominic Dale and Mark Selby the victors.
Dale held his nerve to get the better of Mike Dunn, while World semi-finalist Ricky Walden was O'Brien's victim, beaten 5-4.
World number two Selby survived a scare before beating resilient Scot Ian Burns 5-4.
There were no such troubles for Shaun Murphy, as he beat Michael White 5-1. Elsewhere, Joe Perry thrashed Xiao Guodong 5-0 and Alfie Burden beat Michael Holt 5-2.
Clarke: Oz underdogs
CRICKET: Australia captain Michael Clarke is under no illusions about the challenge his team is facing in the Ashes series.
Clarke, ahead of today's opening Test at Trent Bridge, said: "We come here as underdogs, there's no doubt about that."
Clarke is Australia's only injury worry, and while he declared himself "100 per cent" fit to play, he acknowledges that "my back (complaint) is something I've got to live with every day for the rest of my life."