Where can you get a Gible in Pokemon Ruby?

Answer 1

It is not a selectable Pokémon in Ruby. If in doubt as to which characters are "4th generation" you can consult the Pokédex website.

Gible didn't exist in the original Ruby game. In Pokemon Omega Ruby, however, you can catch a Gible (as a Hidden Pokemon) on Route 111.

Answer 2

You can't get Gible anywhere in Pokemon Ruby because it is a 4th Generation Pokemon that first appears in the Sinnoh region.