Gardner guns, like Gatlings, were the stopgap machine gun for the British armed forces until the development of the Maxim and Vickers guns.  They were the guns that convinced the Brits that they needed machine guns, and they were the guns that delivered on the battlefield.  Sometimes.  

Like any machine, the Gardner could falter, and did in the first battles of the Sudan between Mahdist Arabs and British and Egyptian troops.  The Mahdi ran a rebellion against Egyptian rule of the Sudan along the lines of the Taliban, a rebellian so bloody, the British initially wanted out of the Sudan, and sent their hero, General Gordon, to evacuate Khartoum.  Further actions by the Mahdi, and with General Gordon dragging his feet holed up in Khartoum, led the British and Egyptians to realize their only choice was to let the Arabs have it, which wasn't accomplished till the turn of the century at the Battle of Omdurman.

British Gardner gun

In the Sudan, the British fought in a square formation, with mountain and screw guns, along with Gatlings and Gardners, spaced around it for more firepower.  The cavalry stayed in the middle, ready to ride down the opposition should they appear to lose momentum.  The machine guns lent serious firepower against human wave attacks, which the Sudanese favored.  The Gardners were five barreled affairs, and it is a tribute to the crews of these guns, and the field pieces, that they could keep up with the square when it was on the move, not an easy task with a gun that weighs thousands of pounds.  

The first wave of the Gordon relief expedition was approaching the wells of Abu Klea on 17 January 1885, when the first troubles began with the Sudanese.  Camels were spotted on the dunes ahead, which quickly revealed themselves as the outriders of the Mahdi forces.  After hours of being shot at by ghosts, the Arabs swarmed the square, their pressure causing the square to break up.  The Gardner gun crew found themselves on the outside of the square, when their gun jammed.  (This same gun had jammed earlier at the battle at Tamai, where the Arabs also penetrated the square formation)  The square was breached, and the entire crew of the gun, save one, were killed.  With the Arabs in the square,  only disciplined volley after volley from the Brit's Martini-Henrys saved the day.  None of the Mahdi's men who had entered the square left it alive.

The Battle at Abu Klea

In March of 1885, a British waystation was swarmed by Arabs at Tofrek, killing hundreds of Brits and Egyptians.  There were 4 Gardner guns present at the time, but only 2 got into action.  They appararently shot up everything, Arabs, camels, and possibly their own men, before the Arabs retired.  A thousand Arab  bodies littered the encampment, along with 700 camels and mules.

Video animation from vbbsmyt's channel