There are many possible reasons why a gardevoir might dislike being around gallade. Some find gallade intimidating because of the level of aggression they naturally display. Some find gallade’s protectiveness off-putting. Some actually feel repulsed by them due to type differences.
And some gardevoir are basically lesbians and don’t care for romantic advances from an all-male species.
In any of these cases, rest assured that a dislike of gallade is perfectly normal and just as common as a love for them among gardevoir. This should really only be a cause for concern if your gardevoir is exhibiting a significant amount of distress about the subject or if her feelings are disruptive to her day-to-day life. If that’s the case, start with communicating with her to understand the root cause and work with her from there.
Best of luck!