If I trade my Pokémon from Red to Gold on the VC, and then transfer it to Sun, what will be the Pokemon's original location?

Solution 1:

I've been able to try this for you. So I caught a Pidgey in Pokémon Red, I then transferred this to Pokémon Silver, then using the Poké Transporter / Bank I was able to move it to Moon (pictures below). Unfortunately it doesn't say what region it was from, exactly.

Of course one of the benefits of transporting Pokémon to Sun/Moon or Ultra Sun/Ultra Moon is they instantly get 3 max IVs and Hidden Ability.

Apologies for the low quality pictures, I was using my mobile phone and 3DS, and also for no picture of Red.

Pidgey named STACKEXCH! caught in Red placed in Box 1 in Silver.

Pidgey transferred to Poké Bank via Poké Transporter

Pokémon moved within Poké Bank to Moon

And finally the summary page, which unfortunately doesn't say what region it came from, only that "Seems to have traveled across both space and time to reach you from a distant land in the good old days" The Gameboy icon indicates it was from Gen 1/2 but isn't specific.