I am both wheat and dairy intollerent so have to be careful. You don't actually need any form of rusk, bread etc in a sausage. I think the idea of rusk is just a bulker - rusk is cheap, meat is expensive. And why manufacturers have to load up sausages with milk protein etc beats me. Just mix up the venison, a good bit bit of fatty pork and herbs and spices to taste and make the sausages. Likewise burgers don't need any rusk.
My view on most of the gluten free breads, is that they have the taste and consistncy of sawdust unless really fresh.
Puddledub Pork make any excellent pure meat gluten free sausage and they do even better Chorizo style, that again is pure meat. Traditional Borevors, Toulouse, Sicilian and other recipes all don't use Rusk either.