If I understand your question correctly, you're asking what would happen in the scenario that Giratina tries to use Dragonbreath if it itself has Unown G attached. In this scenario, Unown G does nothing. Unown G's Guard Poké-POWER card text was errata'd to state that Unown G only protects the equipped Pokémon from effects of an attack (excluding damage) from your opponent's Pokémon. It does nothing to prevent effects of attacks to your Pokémon done by any of your own Pokémon (including the Pokémon to which Unown G is attached).
However, if Unown G was attached to the defending Pokémon that Giratina was attacking, the scenario is different. The coin flip still happens. If tails, Dragonbreath does nothing. If heads, the 80 damage applies, but the defending Pokémon is protecting from the paralysis effect because of Unown G.
