To get the Crossover Mission in Roblox Gym-style crossover games, you usually need to unlock it by progressing through the game’s main dungeon/event flow rather than finding it in a separate menu. In the search result I found, the most relevant guide says you wait for a dungeon to spawn, clear it, and then defeat Vermilion at the end to trigger the crossover-related progression.

What to do

  1. Go to the area where dungeons spawn.
  2. Wait for a regular dungeon to appear.
  3. Enter and clear the dungeon.
  4. Defeat the boss at the end.
  5. If the mission is tied to the crossover chain, the crossover NPC/boss progression should appear after that clear.

Important detail

The guide specifically says the dungeon is random and may take time to spawn, with a typical wait of about 15 minutes, so server hopping is sometimes used if you want to try faster.

If you meant a different Roblox game

The term “Gym Roblox” can refer to different games, and one search result was for Gym League rather than a crossover quest guide. If you mean a different title, the exact mission steps may change.

TL;DR

Get to the dungeon area, wait for a spawn, clear it, and beat the end boss; that’s the most likely path to unlocking the crossover mission path in the game I found.