There are several ways you can do this.
If you’re looking to train your gallade’s psychic abilities, meditation is often the key. If you can, try pairing him with a medicham for best effect, but if not, simply be sure to have him sit down and engage in self-reflection for at least an hour a day. While, certainly, it sounds counterintuitive to have your gallade train his mental abilities by not doing anything at all, that’s strangely how many psychics go about fine-tuning their powers. (Some even do so under waterfalls, which may sound like a cliché but is actually an effective way to train discipline.) This is due to the fact that when meditating, a pokémon’s entire focus is on strengthening their inner self and seeking enlightenment, so by stopping all physical movement, they have no choice but to train their minds instead.
Of course, this isn’t the only way a gallade can defeat a flying-type. Gallade can, for example, learn Thunderbolt, Thunder Wave, and Charge Beam, among many others. However, because many of these moves don’t match either of gallade’s native types, training yours to use them may be a bit trickier. On the other hand, the associated training is often less time-consuming. You would still need to dedicate an hour a day to do so, and there will still be some level of meditation involved, of course. It’s just that most forms of type-based training are more active compared to psychic training.
For example, with electric-type moves, while your gallade will need to learn how to channel electricity properly (a process that will involve some meditation, combined with plenty of focus on the field), the bulk of your gallade’s training beyond that will be a simple matter of target practice, as electrical attacks are notoriously difficult to control due to their speed.
In short, each element tends to require different training techniques (some of which may vary depending on whether or not you’re dealing with your pokémon’s native elements), but should you simply want to train your gallade’s psychic abilities, intensive meditation should do the trick.
Best of luck, anonymous!