Referee Stuart Attwell called a halt after 75 minutes at the Broadfield Stadium due to a waterlogged pitch with the tie goalless, and Gregory admitted the decision could even have been made sooner.
The former Aston Villa boss said: "It was the correct decision. It could have been made a bit earlier. It became unfair on the players, as professional footballers, as the pitch became a complete lottery.
"The referee didn't want to stop it but he was aware the game could have gone to penalties and we could have been playing for another 45 minutes."
The game will now been replayed on January 4 and Gregory felt that fans of both clubs "deserved a medal" for turning up on such a wet and windy night.
He added: "Bristol Rovers came into the game in the second half because they had the wind behind them and it became obvious that someone would nick it due to a mistake."