Using NM to 'relay' an internet connection

We have an embedded system that we're working through getting NetworkManager
set up on. There's a mode of operation we'd like to explore, and I'm
wondering if NM has such capabilities 'built-in':
Imagine two units - both have Ethernet and are on the same LAN subnet;
internet connectivity for them is NAT'd via a ADSL router/modem or
such-like, using the Ethernet. 
But Unit A also has a GSM modem, and if the ADSL goes down then it can
switch over to use GSM for Internet. 
What we'd like is, if the ADSL goes down, that Unit B can open 'some sort of
tunnel' to Unit A and have its traffic 'bridged in some way' to Unit A and
then to/from the internet via Unit A's GSM connection. 
It would want to be secure i.e. only Unit B could use Unit A, not anyone on
the LAN.

I use the words 'relay'/'tunnel'/'bridged' in a *generic* sense, as they
often imply specific mechanisms. 

Is this something that NM can provide and manage...? 

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