[HECnet] DECnet Phase IV through Linux GRE Tunnel?

One of the suggestions that was posted here for how one might connect to HECnet was to create a GRE tunnel to a cooperative node, using either a Cisco router or an emulation of one. I don't have any experience with Cisco stuff, and I don't feel enthusiastic about going down that particular rabbit hole just yet if I can find another approach more in line with my experience and interests. I see that there is GRE support in Linux, at least for IP-in-IP tunneling. Do any of y'all know if Linux's GRE support can tunnel non-IP protocols such as DECnet Phase IV? So far I've only succeeded in finding documentation about how to set up IP-in-IP tunneling, and I don't understand what's going on under the hood yet. Since I want to keep my local DECnet traffic local, my current goal is to set up a VAX/OpenVMS simulation under SIMH to act as a DECnet area router (and provide other services to my real VAXen, too). I plan to use a SIMH emulation so I can leave it up all the time to maintain presence on HECnet without running up a big power bill. My real VAX-11/730 and other future vintage DECnet-speaking hardware would only graze on electrons infrequently, and mostly during colder weather.