The top end Gath summon? You get four (I think) Ophanim, and the summoned angel gets four lives (and 4 paths of magic, even if you only use one at a time). The Ophanim make up a lot of the cost difference, and the main angel is pretty nifty in terms of combat endurance. Wish is probably more cost-efficient, even so.
Oops, I just summoned one to see wat they looked like, I didn't get the 4 lifes ability. The spell gives just one Chayot. I guess the 4 lifes stuff compensates the lack of magic diversity, the 55 hps and the extra pearls. Pretty tough against AI's chaff, got to try to hurt him to see his other shapes.