Does anyone know if Gliricidia sepium can grow between Eucalyptus?
Don't know. Do you have a gliricidia to clone from? Simply stick a cutting in the ground and see if it works. No harm done. I would think it unlikely to thrive just because of the nature of Eucs.
Yes: Probably a situation where you are trying to come up with something to furnish it in the first place because it is unsuitable for most kinds of under-planting. Some kinds of trees usually don't have anything growing underneath, gums (Eucalyptus) are an example. One large old one by itself may have other plants come in around it, but a grouping of several - especially if not particularly tall and old, as far as it goes - is likely to be pretty good at staving off competition.