Okay, well, this is a thing. Anyway, there are a few reasons why I think Gaara could beat a Diglett. Most of Diglett's attacks will not be able to get past Gaara's Sand Shield, and his terrarian attacks like Fissure or Earthquake, even though it does have a wide AoE, Gaara will probably avoid that too by leaping into the air or something, as a ninja do. Not even Sucker Punch will be able to do much, because even if it does get past Gaara's precognitive Sand Shield, it would take a lot more than that to get past his Sand Armor. And while Diglett does have access to other status moves like Toxic or Swagger, I don't see Toxic getting past the sand, and the sand does a lot of the attacking automatically, not Gaara, so using Swagger would only hinder Diglett in the long run by boosting Gaara's offensive power.
And then there's Diglett's size to consider. He is one of the smallest Pokémon in existence, and it really would not be too hard for Gaara to catch Diglett and crush him in a Sand Burial. The only advantage I can really see Diglett having is the fact that he can hide underground, but with Gaara's Third Eye in effect, I really doubt Diglett could hide very long. In a battle between these two Sand Manipulators, Diglett is just plain outmatched here.
Winner: Gaara