Gnosis is the raw power of your character. Through the use of Magic, you become more skilled at reaching through the gauntlet to use your power. Gnosis can be increased when the experience is spent.
The level of skill your character has in a particular arcanum is based on your power. Your natural ceiling in an arcana is the highest you can go without training. Training will require role playing.
Archmastery, if we ever get there cannot be taught. It will require an immense amount of study to increase a sphere by a single dot beyond 5.