Allen, whose Gills are still awaiting a victory in League One, saw his side twice come from behind to deny Mark Cooper his first win as permanent Robins boss.
The impressive Nathan Thompson thought he had won it for the hosts in the 68th minute, only to see McDonald slot past Wes Foderingham in the dying seconds.
Allen said: "It is a good day for our club, but we need to keep practising in order to get that first win of the season, keep training, keep believing, keep cool, keep calm and keep on doing what we did all of last year.
"It was a good point, a good team performance - everyone put a real shift in. For the last two weeks all the players have worked extremely hard, mentally and physical, after the big setback we had.
"You have to give so much credit to the players, they have been focused and today was a good result for our supporters, who never stopped singing from the beginning to the end.
"It has been a step up in the quality of play this year, the clubs and teams we're playing against makes that quite obvious, but overall we defended very strongly and very resolutely as a team."