For a long time, Arabs have been following the Egyptian style of transliterating the hard G, by using the letter ج for it, because Egyptians pronounce the ج as a hard G.
The Arabic language does not really have a letter for the hard G, even though it is widely pronounced in colloquial Arabic instead of the letter ق or soft ج.
The current trend, particularly in the eastern Arab world, is to use the letter غ instead of ج for the hard G.
I think this is a much better practice, because ج causes ambiguity; it can be either a hard G or a soft G—there is no way to tell. By contrast, the letter غ will rarely cause ambiguity because it does not exist in English and most other foreign languages that are translated into Arabic. The typical Arab reader knows that there is no غ, and will therefore know that the غ is actually a hard G (e.g. غارث is Garth and not Gharth).
I have heard some Arabs pronounce Chicago as "Shicagho," because they see it spelled شيكاغو but they are only the uneducated. Had it been spelled شيكاجو, they would have pronounced it Shikajo, which is even worse.
Here are some cases of ambiguity that result from using ج rather than غ in transliterating the hard G:
When Arab readers read the name جوردن they have no way of telling if it is Gordon or Jordan.
When they read the name جيري they have no way of telling if it is Gary or Jerry.
When they read the name جيل they have no way of telling if it is Gill, Jill or even Gale.
When they read the name جينزفيل they have no way of telling if it is Gainesville or Janesville.
Also, using ج rather than غ results in pronouncing Peggy بيجي as Pejjy, Indigo إنديجو as Indijo, Gilmour جيلمور as Jillmore, Arlington آرلنجتون as Arlinjton, and Asperger أسبرجر as Asperjer, which all sound silly. Or if readers assume that the ج is a hard G, they will pronounce Bridgeton بريجتون as Briggton, Woodbridge وودريج as Woodbrigg, Edgewood إيجوود as Eggwood, Judson جدسون as Gudson, and names like Ranger رينجر and Geronimo جيرونيمو with a hard G.
All of these ambiguities can be avoided by designating غ for the hard G and ج for the soft G.