Just saw this now. Yes, Plynkes is correct Re: Gatling guns on the NWF, they were always crewed by the Royal Artillery. Empress have a RA Gatling in their Zulu War range. The crew are in blue service dress uniforms but can easily be painted khaki. On the Frontier they would generally -- though not always -- be wearing puttees, but for a "What If?" scenario I don't know how important that would be. There are other Gatlings on the market, such as in the Perrys Sudan range, but they have Naval crews. I don't know any example of Naval gun crews or Naval Brigade Infantry serving on the Frontier, though during the Mutiny maybe some got close.
Depending on the specific date of your What If scenario, there is a British Maxim gun and crew in the old Wargames Foundry Boer War range, wearing khaki uniforms and puttees.