Gallacher pulled off great saves from Murray Davidson and Chris Millar while Steven Milne missed a sitter as St Johnstone had to make do with their third draw of the season.
Jody Morris had driven home for the visitors four minutes after Billy Mehmet's penalty gave St Mirren a 33rd-minute lead.
The burst of action came out of the blue following a scrappy opening. St Johnstone began to find an edge midway through the first half as they threatened from several set-pieces, one of which prompted a penalty appeal following Hugh Murray's clumsy aerial challenge on Davidson. Referee Steven Nicholls waved play on and Graham Gartland sliced a volley wide from the loose ball.
The hosts were unchanged from the team that won at Kilmarnock but it took 29 minutes for them to have a shot at goal when Andy Dorman's free-kick from the left landed on top of the net.
But they soon had a far easier chance when Stephen McGinn won a penalty when he beat one man and went down after taking the ball around Dave Mackay.
There appeared to be contact but Mackay angrily accused the Scotland Under-21 international of diving, earning himself a yellow card, before Mehmet sent Alan Main the wrong way.
The visitors tried to hit back immediately and Filipe Morais curled a free-kick straight at Gallacher. Main dived to push away Michael Higdon's header following a Dorman corner before the visitors levelled.
Danny Grainger's long throw was cleared to the edge of the box and Morris drove into the bottom corner with Gallacher unsighted. The goalmouth action continued and Morais shot just wide of the far post with the aid of a deflection after going past David Barron inside the box.
St Johnstone should have taken the lead 60 seconds after the restart when Morais broke clear down the right and produced a brilliant cross but Milne headed over from three yards.
Barron had a curling effort saved at the other end but St Johnstone, with Millar on for Paul Sheerin, were using the width of the pitch to push their hosts back. St Mirren manager Gus MacPherson brought on Craig Dargo for Higdon in search of a spark in the 61st minute.
The spark never materialised, although a late tackle by Collin Samuel on Murray prompted the second touchline argument of the game before MacPherson and opposite number Derek McInnes shook hands.
St Johnstone were pushing midfielders forward and Davidson's glancing header was saved by Gallacher following a cross from Morais, who was soon replaced by Martin Hardie.
Davidson almost won it in the 72nd minute when Gallacher touched his header on to the underside of the crossbar after Hardie had headed back Samuel's deep cross. Gallacher then pushed Kenny Deuchar's downward header past the post after the substitute had attacked Grainger's cross.
Grainger and Dargo were booked after squaring up to each other following a foul by the home striker, before St Johnstone again failed to capitalise on a superb chance.
Milne cut the ball back for the unmarked Millar eight yards out but Gallacher blocked his first effort and the midfielder headed the rebound against the crossbar. The home keeper had time for a more routine save, when he dived low to catch Gartland's header from a Grainger corner.
Referee Nicholls angered the hosts in injury-time when he blew up to book Gartland for a foul on sub Tom Brighton as McGinn strode forward with the ball at his feet.