The chance of a Gym Leader visiting is still unknown seen as Nitendo has revealed it.

The Rank of your Avenue has no effect on when a Gym Leader will appear neither will the amount of visiters or time your Avenue has been open. Ofcourse if you have had Join Avenue open for a while you have more chance.

Although, some people think that the chance of a Gym Leader appearing is the same chance of encountering a shiny.

Personally I disagree with this but...meh.