The Greek, Etruscan, and Roman alphabets (in that order) took on the Semitic order and kept it basically unchanged. This makes sense for Semitic and Greek where they were used as numerals, but it's just tradition in the case of the Romans. When they dropped zeta, the sixth letter, as unneeded, that left a gap in sixth place. So when they invented G by sticking a bit on C, they out it in the first available gap.
The original Semitic order I can't explain, and have never heard a theory on it, but I believe it was pretty rigidly adhered to across the different Semitic peoples that used it over the ages.
The modern Arabic alphabet has changed it by grouping together all the letters that are identical apart from dots (such as B, T, and Th), but even they still use the original Semitic when using them as numerals: i.e. the letters now go A B T Th J H Kh D Dh... but they number things as (a) (b) (j) (d)...
