My suggestion is to update your BIOS, change to Legacy boot if currently UEFI (if possible), or switch the physical slot your GPU is in and see if the problem goes away. The issue is your build parameters, not my plugin. The only other option would be to figure out a way to squelch that log.
Maybe a combination of using this script and creating a custom .conf file to remove the offending log messages. If anyone wants to try it, create a file (02-barmappingblock.conf) in /etc/rsyslog.d/ that contains:
:msg,contains,"mapping multiple BARs" stop
:msg,contains,"which spans more than PCI Bus" stop
Then do a:
/etc/rc.d/rc.rsyslogd restart > /dev/null
If it works, then copy that .conf file to the location the script suggestions, and then add it to your startup scripts when UnRAID boots. It's not a 'fix' per se, but a kernel/driver issue can't be fixed by myself or the linuxserver folks.