The Glaseado Gym is home to the Ice-type Gym Leader Grusha.
To pass the Gym Test, you need to complete the Snow Slope Run within the time limit. Leave the gym and go up to the Pokémon Center, and talk to the gym staffer near the snowman to start the Gym Test if you're ready.
You will slide down the slope on Koraidon/Miraidon. You will automatically slide down and can't go backward, but you can move the left stick downward to go more slowly and move it upward to go more quickly. You need to reach the finish line within a minute and a half (1:30). You will get a time penalty if you don't go through the colorful arrows as you go down, so try not to miss too many.
When you're ready, go back into the gym and talk to the gym staffer, and say Yes to battle the Gym Leader.
Ice Gym Leader Grusha, the Sub-Zero Shredder
Grusha has a level 47 Frosmoth, a level 47 Beartic, a level 47 Cetitan, and a level 48 Altaria that he Terastallizes, with Tera Type Ice. You get 8640 Pokédollars for winning.
You will get a Gym Badge. Check the Gym Badge Effects page to see the level of Pokémon that will listen to you and be easier to catch, or open the Map App or Pokédex App and press X, then select Profile, then press A to see the effects of your current badges.
You also get a TM124 Ice Spinner.
A new Gym Badge means new sandwich recipes. Be sure to stop by any Every Wich Way and talk to the person in front of the counter to learn some new recipes.