Now you got me thinking of this as being similar to the "who is Ash's father" debate! Great job folks! :P [/sarcasm]
Though this sounds as if it's one of those questions that we're not supposed to know the true answer to, I do, as CodeHelmet does, believe that Gary/Blue could be one possibility as to a post-Giovanni Viridian Gym Leader, given that Blue was the Gym Leader there during the Generation II mainline games (and the remakes in Generation IV).
Or, perhaps, that plot line involving the leadership of the Viridian Gym post-Giovanni WAS intended to be revealed at some point in the main anime, but was pushed off to a future side series or even movie installment (though the movies are now doing an "alternative universe" telling of Ash's story) - just as the GS Ball story was supposed to center around Celebi, but, of course, that plot was scrapped due to Celebi being the star of the fourth movie.