im not too sure either but i think that gateways can be tied off, IIRC Ishameal ties off his gateway to TAR or where-ever in TDR when fighting Rand in the Stone. i think Ravhin did the same thing in FOH. i don't think Moggy mentioned it to egwene tho, otherwise all travel capable aes sedai could do it. on the other hand tho it might only work with TAR traveling as you technically don't leave the place where the gateway is and is impossible to do when crossing 'the real world'.

a permenantly open gateway would no doubt have a negative effect somewhere along the line, it could put a strain on the pattern after awhile, like the 'windows' made by the Subtle Knife in His Dark Materials did to the different worlds and Dust.