Graendal opens the gateway before shielding them. Several posts have gotten the two out of order.

I doubt Aran'gar was holding the power, though I still believe this scene is a testament to Graendal's skill. Aran'gar, however idiotic, is still a powerful and skilled channeler. Graendal's gateway-shield-Air bounds must have taken less than three or four seconds, otherwise I believe Aran'gar could have reacted with more than words.