The Earth Girl Arjuna OST 1 probably has the most Gabriela Robin material found on any Yoko Kanno album. But sadly, not all songs played in the animation was published. I guess there was a surplus of so many songs… The unofficial fan-ripped album created by roxfan of YKDB contains 47 background music tracks in total, and a couple of notable Gabriela-sounding songs in it.
UPDATE: Download the tracks here. These are available through RapidShare, so these links will expire if no one downloads them for a while: