The late 30s and early 40s were a golden age for adventure movies, what with the rise in budgets during the economic recovery, the changes to screen entertainment since the production code became enforced and the general carefree optimism of the times. While most of these were rip-roaring swashbucklers about the wild, superhuman and often frankly misogynistic exploits of heartthrobs like Errol Flynn and Tyrone Power, Gunga Din is very different in its focus, scope and tone.
Part of Gunga Din's secret is the division of labour in its writing team. The original story is by Ben Hecht and Charles MacArthur, two of the most skilled and celebrated writers of Hollywood's golden age. However the actual screenplay was the work of Joel Sayre and Fred Guiol, both of whom, Guiol especially, had a background in comedy. What we get from these four is a plot that is well-balanced and engaging, yet also cleverly spiced up with comical touches. Most of the adventure flicks of this time were at least partly comedies, usually featuring one or two comic-relief supporting players, but they didn't use laughs in the way Gunga Din does. Here, all the main characters are capable of being objects or originators of jokes. We see the sinister menace of the bad guys suddenly diffused as the scene dissolves into a light-hearted brawl. The first main battle scene is an even-handed blend of action and gags, in the style of the silent swashbucklers of Douglas Fairbanks, Sr., something which the Flynn and Power vehicles largely failed to replicate. Towards the middle of Gunga Din the action must necessarily take a break and there are lots of talky scenes for the sake of the plot. However the continual forays into comedy  such as the spiked punch routine  make this "slow" portion bearable.
Producer-director George Stevens was a natural when it came to this sort of thing, himself having cut his teeth at the Hal Roach studios, and almost exclusively having directed comedy up to this point. This was his first full-on action feature, and he does a startlingly good job. In particular his use of moving point-of-view shots make the battle scenes extra exhilarating. He also brings something you seldom see in action pictures of this era  a sense of real dread and fear. He sets this up with those stark and foreboding mountains dominating many of the shots and dwarfing the characters. The portrayal of the abandoned village and the Thuggee cultists cry of "Kali!" is genuinely haunting. This dimension of fear plays into all the other emotions that are at work here, causing us to worry for these likable characters, and making the comedy a greater relief of tension.
A real touch of genius is in the way the eponymous hero is introduced to the audience. We are made aware of Din visually, as he is prominent in a number of scenes before any of the characters actually address him or verbally refer to him. Because of this, we are given the impression that Din is not an important figure within the regiment, but he quickly becomes a notable character to us, and crucially a sympathetic one, as we see him risking his life and giving water to dying men.
But the best efforts of writers and directors are all for nought without a capable cast. Fear not, for Gunga Din has a top-notch one! Victor McLaglan and Cary Grant were ideally suited to the material, since their best roles were generally found somewhere on the spectrum between drama and comedy. Grant in particular is at his best, largely believable but just occasionally breaking into that over-the-top whooping and capering that was his trademark. Douglas Fairbanks, Jr. is not quite up to the standard of his heavyweight companions, but he is by no means bad. And of course there is Sam Jaffe, cursed by his looks to forever play these wizened little oddballs, but who else could play them with such dignity and humanity? I have not set out to bash the swashbuckling adventures of Errol Flynn and Tyrone Power, and indeed many of their pictures are absolute classics that I love absolutely. But Gunga Din does things that even the best of those swashbucklers could never achieve. Not only does it dispense with the dashing male lead or the clichéd defiant damsel, it successfully merges the action genre with comedy and poignancy, in a way that few pictures have done before or since. And that's fabulous.

While the acting and directing could be argued as having some merit - the storyline is a very poor wannabe Vietnam movie with the country name simply changed.
At the very least, for a movie to hold some credibility, try and have some semblance of accuracy in equipment, weapons and tactics. Nevermind the gross misrepresentation of the behaviour of the troops as a norm.
Aside for the limited use as silly propaganda about the South African Defence Force, it serves little purpose - definitely no entertainment value.
Aspiring movie makers - this is how not to make a war movie. Do some research, and have some pride in your product.