G, J, K, U, W, Y, and Z are all characters that aren't used in Latin (in the case of G, J, U, and W, because they didn't exist; in the case of Y, it is used in Greek words but doesn't represent a sound that's possible in Latin; I'm not sure if Z was used in Greek words or not. K is just a Greek letter -- since it represents the sound written with C in Latin, it is never used at all).

Sure about the G?

Hardly.

See https://la.wikipedia.org/wiki/Gaius_Iulius_Caesar

Latin wikipedia isn't exactly an ancient source. Without bothering to read the article, why do you think it has a subhead of "Fontes_de_vita_C._Iulii_Caesaris_praecipui"?

I'll take this cum cranulo salis.

You think "cum granulo salis" is evidence for G being an original part of the Latin alphabet, while not being evidence that U was an original part of the Latin alphabet?

Sure, G wasn't "originally" part of the Latin alphabet—until they invented that letter.

What do you think makes G different from J and U in that regard?

Timing. Where do you think G slots in the timeline, relative to the Latin classics?

G dates to the classical period. But, as I pointed out above, it's still intrusive enough then that Caesar's praenomen, Gaius, is written C, not G. The name is older than the letter, but younger than Latin writing.