They don't have to provide the sources forever...
Seemingly the model in question (WAG200G) is originally from 2007.
Excerpt from the GPL [1] (paragraph 6b):
"You may [...] Convey the object code in, or embodied in, a physical product [...], accompanied by a written offer, valid for at least three years and valid for as long as you offer spare parts or customer support for that product model, to give anyone who possesses the object code either (1) a copy of the Corresponding Source for all the software in the product that is covered by this License [...]."
[1]: http://www.gnu.org/licenses/gpl.html