It does not mean donor. It's just a shortened form of Donarghe, which is the fictional criminal organization of the Bravan series.
By the way, Donarghe is the spelling I made up. The katakana spells "dona-ge", but I didn't want to spell it "Donage" or "Donahge" because you wouldn't know how to pronounce it. And given that Gel operates out of the New York City branch I decided that Donarghe may have a soft R sound, much like the katakana "ka-" means car. But I still didn't want to spell it Donarge becuase you guys would still pronounce it in two syllables like "doo-nahrge" or "doh-nahrge" when it should be pronounced in three: "doh-nahr-geh". If you make a nor sound (like donor) instead of nahr, you're pronouncing it wrong.