The former Stockport and Motherwell boss is at present with Posh only until the end of the campaign but at the club's annual meeting, earlier in the day, owner-chairman Darragh MacAnthony confirmed he had made Gannon an offer to stay permanently which he is now considering.
Bidding for a third successive straight win, Posh had the better of an opening half that produced some refreshingly open play but failed to make the most of several well-worked openings, particularly when Craig Mackail-Smith headed over after Exdous Geohaghon nodded across a Sergio Torres corner.
Argentine wide man Torres, back in favour at London Road after a loan spell at Lincoln City, should have broken the deadlock when released by Tommy Rowe only to fail to get the ball past well-positioned Neil Sullivan.
The veteran Rovers keeper, who celebrated his 40th birthday last month, was even busier after the break, twice making finger-tip saves to keep out well-placed headers from Charlie Lee.
The hard-working Posh midfielder finally beat Sullivan with another header on 55 minutes, although it was a close thing with the ball bobbling on the line before referee Fred Graham confirmed his assistant's signal that it was a goal.
Dangerman Billy Sharp blazed wide as Rovers tried to hit back straight away and Sean O'Driscoll's side did haul themselves level 11 minutes later with skipper Gareth Roberts beating Posh's Joe Lewis with an unstoppable angled right-footed volley from 20 yards.
The game could have gone either way in an often frantic final ten minutes with Mackail-Smith denied by a one-handed Sullivan save before Rovers had the last word with Oster's 90th-minute shot flashing past Lewis.