does application.loadlevel run Garbage Collector automatically?

Questions & Answers
1

AS the question says, does loading a new scene automatically clear garbage?

2

Yes. loadlevel uses this process:

  1. Load all new assets required
  2. Load new level GameObjects
  3. Destroy previous level GameObjects
  4. Awake new level GameObjects
  5. Clean up unused assets

If you want to load level without destroying objects you might want to use Application.LoadLevelAdditive
(Unity - Scripting API: Application.LoadLevelAdditive)