You’re welcome, anonymous.
To answer your first question, you may wish to try the Celadon Gym. They may specialize in grass-types, but they’re willing to work with any trainer and pokémon who needs help. They may also recommend other fitness centers in the area that specialize in physical fitness for delicate psychic pokémon such as gardevoir. (My own knowledge is unfortunately limited, not only because I’m not local but also because, when I had been … I wasn’t exactly familiar with local fitness centers, to put it lightly.)
As for restaurants, most are generally diabetic-friendly. Just be sure to choose lighter options and skip dessert.
Finally, regarding ice cream, you may not have to skip the real thing. Pay attention to nutritional labels and choose ice creams with lower total carbohydrates. This translates into lower sugars, so the lower your carbs, the better. Additionally, you’ll still need to be aware of her blood sugar levels and what she’s already eaten before giving her ice cream. Of course, don’t forget the portion size. Let your gardevoir enjoy ice cream, but don’t let her overdo it. After all, just because you have a dietary restriction doesn’t mean you have to give up the things you love; you just have to do a bit more self-checking and a lot of label reading before you jump in and indulge. And believe me: as much of an annoyance as that sounds, all of that is actually not that bad once you get used to doing it.
Best of luck!