You gain +1 charge of Mana Flux.
Gathering Power is not a subtle process. For every Action you spend Gathering Power, you must make an Arcana check (with your Primary Arcana skill) against the Perception DCs of any observers/listeners to control your magical emissions and avoid their notice.
See the Gathering Power section for rules on how much Mana Flux you can hold and how you can use Mana Flux.