Does Unown G's "GUARD" Poke-POWER prevent effects from attacks that the Unown G is attached to?
No, the Poke-POWER only prevent effects from an opponent's attacks, not your own. Unown-G's "GUARD" Poke-POWER should say, "As long as Unown G is attached to a Pokemon, prevent all effects of attacks, excluding damage, done to that Pokemon by your opponent's Pokemon".
Source: PUI Rules Team (2008-02-21), Organized Play News (2008-02-28)

Relevant cards

Note: All applicable cards may not be listed below.
DP4_57