The Giga Parasite is a Soul of Rebirth enemy from Final Fantasy II. Its attacks drain MP. This can be useful as it can be manipulated to increase your party members' max MP levels, especially Minwu's.
Giga is a unit prefix in the metric system denoting a factor of a (short-form) billion (109 or 1,000,000,000). It has the symbol G.
Parasitism is a non-mutual relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host.