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.

Also, the fact that Nynaeve did not detect the presence of saidin exemplifies her lack of formal training. She knows something is different - she says that perhaps it is more subtle - but doesn't put one and one together.