I know I QC checked this months ago, but i'm afraid I have to reject this analysis now. At the time, Gardevoir could check Rain sweepers (which was a pretty big deal) but that niche is gone. I guess it can check Excadrill, but even then thats nothing special. Its relying on Focus Blast of all things to check him, and you're choiced too. Gardevoir is also thoroughly outclassed by other Psychic types (Lati@s, Reuniclus, Mew etc)
In retrospect, I agree with you. Had Aldaron's proposal not been passed, and if Gardevoir had base 90 speed, then it would have an excellent niche in OU, but now it's just not worth running. Sun teams are nowhere near common enough to warrant her presence on a team, and Porygon2's access to Eviolite only worsens her case as a defensive Trace supporter.