I suppose you could classify it as a fumet, but it's pretty intense for that -- and way past a court buillon. You could cut it with some water, I suppose. The problem with most broth from bottled gefilte fish is that the manufactuer boiled it, so it often carries a little bittnerness.
Other than just saucing the gefilte fish quenelles, It's highest use is probably as an aspic gelee garnish -- clarified with a raft, mixed with a little gelatin, chilled, and cut into decorative shapes. It typically has enough protein that without any additions, it will gell with a little chilling. You won't get it really brilliant without clarifying it though.
If you're doing seder for the gonsom mishpucha it's worth making your own gefilte fish -- and probably then and only then going through the work to make a proper aspic. Otherwise, my sense of proportion (twisted though it may be) tells me to buy gefilte fish on sale while they're reducing the old Passover stock, and just use the broth as is.
BDL