I cannot recall an instance of them being tied off, off-hand, and do not have access to my books, but all I can recall are cases where the one who wove the gateway holds it until everyone passes through. This is the source of Grady's problem in tGS - he lacks the strength to hold a gateway open long enough for Perrin's entire party to pass through. It seems that the obvious solution is to tie a gateway off, as RJ has said that such weaves are perpetual (unless the weaver gives them an expiration date), and once tied off, no longer draw on the channeler's strength. As far as I can see, this means one of three things:
1. Gateways cannot be tied off.
2. Grady cannot tie off his weaves (back in the early-mid books, it is suggested or stated that this is a rare skill).
3. Grady and/or Perrin lacks the imagination (or is simply too tired) to think of this solution.

Is there any evidence one way or another that anyone is aware of? Either a statement that the gateway must be maintained, or an instance of someone tying off a gateway, even temporarily?

I do not count Aviendha's gateways in this BTW. Her first one was not tied off, it was left to close on its own, and Rand blocked it. Her susequent gateway in Altara may have been impeded by her block against weaving gateways, and/or her intention to unweave the gateway, either of which could be a reason why she did not tie it off, regardless of whether such a thing is possible.