To come back to Gaul: Even though "explaining the weighting system which drives the production AI in full is beyond the scope of this document as it would take several days to write", it is quite clear to me that Gaul only builds:

Temples to Abnoba and temples to Teutatis (unmodded), as a Religious Smith.
I have yet to witness any RTW player stating otherwise.

This leads me to believe that there is no chance involved when it comes to temple choice (it's exclusive: ie choose one god and stick with it). If it isn't, ie random rolls, then these are triggered way too often to allow for the other temples to Epona or Esus.
This means that the chance roll favoring temple A or temple B ensures temple A and B get built. Unmodded, you would have to give Gaul close to 40 Regions in order to begin seeing 1 to Epona/Esus, and even then odds are probably against them, similar to AI re-checking and re-establishing all of its troops every time you reload.

A further corollary is that it is impossible for the Julii to get their Panthenon to Epona (+5 XP, the best battle-temple there is in my opinion), with an unmodded version of the game. So you have my leave to give the Julii the option to build the first 3 levels of said god.