That it was a saidin Gateway, and it seems very unlikely that it was made by Moridin.

I think this is also shown by Graendal's thoughts, that she figures that the messenger must have been sent by Moridin, because only he knew where to find Graendal:

"A messenger in black strode through the portal, and she knew what he wanted before he spoke. Only Moridin knew where to find her, now that Sammael was dead."

If the Gateway was made by the TP, Graendal would already know for this reason that Moridin calls her (besides, as you said, TP Traveling is different). Plus, Graendal's statement is only correct, if she meant after seeing this saidin Gateway, that Moridin is the only male Forsaken, who knew where to find her. After all, Cyndane and Moghedien visited Graendal's palace as well. Therefore I believe that Graendal would have been surprised, if it had been a saidar Gateway.

I suggested before that Moridin's beautiful messenger might be Osan'gar Reborn. But BS indicated that Osan'gar wasn't reborn. Apparently the messenger is still a channeler, though, or maybe he had at least special ter'angreal to make a Gateway. It was also suggested that the guards in black uniforms, which Graendal sees outside of the Tower and who seem unaffected by the heat, might be Asha'man, since normal guards probaly couldn't deal with dangerous Shadowspawn.