Anything that is part of the attack done to the Pokemon with Unown G attached. In this case, the damage part of the effect is excluded from Unown G's prevention.
Examples of non-damage effects might include things like: placing damage counters (e.g. Bannette's Ghost Head), applying a Special Condition (e.g. poison), switching out, reducing defending Pokemon's attack damage on next turn (e.g. Meganium's Delta Reduction).