Knowing just the Nature and the Personality of a Pokemon doesn't give you enough information to determine any real specifics about a Pokemon. You can really only make general guesses.
In this case, a Gentle nature will reduce Defense by 10%, but will also increase Special Defense by 10%. Competitively speaking, this is one of those "never use ever" Natures, because it reduces Defense. Ideally, you always want to reduce the offense stat you're not going to use, or in a Trick Room team to reduce Speed (which effectively increases it instead).
The Personality Trait tells you which stat the Pokemon's highest IV is in, or is tied for. "Highly Persistent" corresponds to Defense. But that's all the information that it gives you. It doesn't give you any indication of how high, just that it is the highest or tied for highest. Meaning it could be a 31. Or it could be a 12. It's just generally not a very useful piece of information.
We do kind of get lucky here, though. A Gentle nature and a Highly Persistent personality both affect the Defense stat. This means that this particular Pokemon is probably not a good choice to use for battling, because it's stat that has it's highest IV is also reduced by 10%.
