I think I have the answer to this: the Umbaran Ghost Nebula is the source of green blaster gas. The opening narration's mention of 'supply routes' implies some kind of funding being provided by the Umbarans, but given the planet's extreme defences (both the Umbaran militia's, with their advanced weaponry and the sizeable Separatist defence fleet) make this also seem like not enough reason to launch such a costly invasion.

The battle of Umbara however is somewhat unusual in that is is a Republic invasion of a world that did not call for help and does not appear to be a major Separatist base, with no known droid foundries or anything similar. It is seperated from the rest of the galaxy due to its location in the Ghost Nebula, a cloud of gas which somewhat limits communication and travel in and out of it. As you may know from the highly memorable Umbara arc of The Clone Wars, Umbara is a planet in the Expansion Region of the Star Wars Galaxy, stated by the opening narrator of The Clone Wars to be ' strategic '.

Star wars battlefront 2 umbara