I do not think that the SME server gateway is made for running in bridge mode.
On the other hand I think any Linux distro including the sme server gateway can be modified or reconfigured to run in bridge mode. (But I do not understand why it should be an good idea to run the SME server gateway in bridge mode.)
I have tried to set up Centos to run in bridge mode, and I also made a small guide about it for later reference and use.
On the other hand I do not understand at all what it will mean to:
..use SME Server 7.1 in "bridge" mode (proxy filtering users) like Cencornet do..
Because of this it would be interesting to read some info about Cencornet or in some other way to get some info to understand how things are expected to work.