Following his schooling in Edinburgh where he demonstrated early skills with the pencil, he began to attend drawing classes at the Board of Manufacturers' School and the Life School of the Royal Scottish Academy. It was at the Academy that the 25 year-old artist exhibited his first painting in 1867 - an Arran landscape. This would be the first of no fewer than 143 paintings by him exhibited at the RSA during his lifetime. However, it was not landscapes that he was to make his reputation but with figure studies, and some of his earliest work focused on great events in his nation's history, 1874 saw his painting of "Columba in sight of Iona" followed two years later by "The Death of Columba".

His early interest in pictures illustrating Scottish history and portraits of eminent men gave way to a focus on military themes particularly depicting the Scottish soldier in battle. In 1887, his first military picture, Comrades, was exhibited at the Royal Scottish Academy (RSA). Gibb had been studying the life of Naopleon and had made a sketch of the retreat from Moscow. On seeing the group of three soldiers in the foreground, one of whom had fallen in the snow, a colleague suggested that this vignette would make a suitable subject for a composition. The result was Comrades, where a young soldier whispers his last message to a comrade who comforts him amidst the snow in the Crimea, both being guarded by a stalwart highlander. The following year, Gibb exhibited a finished picture slightly modified of the The Retreat from Moscow. This work was transposing the Scottish soldiers with Napoleonic figures. Behind the column of French soldiers trudge wearily through the snow into the distance. A similar picture to Comrades was his 1884 piece, Schoolmates depicting two highland officers in the heat of battle, one falling wounded into the arms of the other.

Perhaps his greatest work appeared in 1881. Entitled The Thin Red Line, it was inspired by Alexander Kinglake's account of the 93rd Highlanders at Balaclava in his book The Invasion of the Crimea. Gibb had taken a walking holiday in the English Peak District and reading Kinglake's book, and while out walking near Haddon Hall, he glanced up to a slight rise and imagined that he saw a line of highlanders "all plaided and plumed in their tartan array," He quickly returned to his lodgings and sketched his mental image. The result was one of the finest military paintings of the nineteenth century. The picture represents a line of 93rd Highlanders stretching along a slight rise as Russian cavalry come into view. It was a great success causing much excitement at the Academy exhibition and it earned for its creator full membership of that renowned body in February 1882. As one reviewer wrote, "The features of the men and the incidents of the scene are given with competent detail, yet the picture generally is touched in a free and broad manner; and while the actual horrors of war are not prominent, the strained attention of the troops, and the few incidents on the right or Russian side of the picture, give the scene all the intensity it requires. The picture was shown at the Royal Academy the following year and thereafter became the property of Archibald Ramsden who also acquired several of Gibb's other important military paintings. Following the death of Ramsden it was sold at Christie's on February 1, 1917 to Sir Thomas Dewar for L882 although representatives of the regiment also bid on it.

Kinglake's history also provided the source for Alma: Advance of the 42nd Highlanders which was exhibited in 1889. The author described how Sir Colin Campbell at the head of the 42nd rode up to the regiment and uttered the following "Forward, 42nd!" This painting was based on details provided by Colonel Sir Peter Arthur Halkett, who carried the Queen's colour in the Alma and represented Lieutenant Colonel Sir Duncan Alexander Cameron who commanded the 42nd and Sir Colin Campbell on horseback urging on another "thin red line" up the height of the Alma toward the distant enemy, although Gibb did include two dead and one wounded Russian in the left foreground for effect. The final picture of Gibb's Crimean scene was Saving the Colours: The Guards at Inkerman, which was painted at a temporary studio set up on the slopes of the Pentland Hills, although his 1885 canvas, Letters from Home (now destroyed) also takes as its subject a scene in the Crimea with two officers in studio built to achieve the appropriate effect, soldiers from the Edinburgh garrison were brought to the site to serve as models. One wonders whether the Pentland Hills also served as the inspiration for the heights of Alma in his earlier work. The finished canvas depicted Lieutenant H.W. Verschoyle holding proudly aloft the battered flag of his regiment while the Duke of Cambridge can be seen in the right background welcoming the Guards.

Throughout the last decade of the nineteenth century, Gibb continued to paint portraits of important Scotsmen particularly clerics and academics but his reputation was built on his military paintings. He returned to war for his 1903 painting Hougoumont, 1815 which turned out to be his last retrospective military scene. Following the battle of Waterloo, the Duke of Wellington had written "the success of the battle of Waterloo turned upon the closing of the gates of Hougoumont," a chateau situated on the west end of a low ridge that ran across the battlefield. The building was also in front of Wellington's right. At 11.30 a.m. on the morning of the 18th June 1815, the French attacked La Hougoumont but the garrison held on. The painting represented Colonel Macdonell, Lieutenant Colonel Wyndham, Ensigns Harvey and Gooch, Sergeant Graham and Corporal Graham all of the Coldstream Guards, and Sergeants Fraser, Bryce, McGregor and Alston, and Private Lister of the Third Guards in the act of closing the gate against the onrushing French.

Gibb thereafter focused on the contemporary military events, with Dargai, October 20, 1897 painted in 1909 the year after he became His Majesty's Painter and Limner for Scotland, and two pictures from the Great War. With Dargai, Gibb again represented soldiers at the most critical point in battle, in this case, the dash of the Gordon Highlanders across the fire-swept zone below the heights of Dargai. For the next eight years, the artist did not paint a single military scene, preferring instead the secure income from portrait painting sprinkled with a few scenes from Italy and Egypt where he had spent several months sketching in 1911. Even the outbreak of war in 1914 did not stir the 68 year-old artist to commit to canvas images of battle, although he did venture back into the subject with his Communion at the Front shown in 1917 so that it could be presented to the British Red Cross Society for an art sale to benefit their war work. However, it was to be another 12 years before his final military picture appeared. In Backs to the Wall, 1918 the artist relied on his well and trusted compositional effect of a line of soldiers at the critical moment, in this case a line of khaki-clad Scottish troops standing defiantly, bayonet at the ready.

Gibb died at his residence in Edinburgh in 1932. Tributes poured in, particularly from his colleagues at the Royal Scottish Academy where he had devoted so much of his time including his 12 year as keeper. But today he is remembered for his stirring battle scenes however anachronistic they may be, in which he demonstrated his concern with soldiers at the moment of their greatest trial,  Peter Harrington