In my case, I'm looking for pictures of male Gardevoir, but there's no male_gardevoir tag, and just searching "gardevoir male" doesn't narrow the results very well. This site seems very good about tags and searchability, so I'm guessing there's a way to do this kind of search, but I can't figure it out. Can anyone help with this?

These are how I would try searching. None of these are 100% fool-proof, but should really narrow down closer to what you're looking for.

gardevoir male solo narrows to only lone male Gardevoir
gardevoir solo ~male ~ambiguous_gender same as above, but includes the chance for ones that aren't clearly any specific gender.
gardevoir male -female -intersex may include male partners