Just got gprs working with an old phone, enough to (slowly) upload a photo of the LimeSDR to Facebook
I’m running all on one box with USB3 to the radio and ethernet to a nat gateway, Ubuntu 16.04 - so here are all my config files that work. There are tricks, like the example cfg in the source have different unit-id’s for the bts and the nitb bsc, plus the aforementioned rx-gain is 1 and needs cranked up to 43 or so.
My last challenge was disable the ufw (uncomplicated firewall), flush all rules out of iptables and load the one nat rule, then it started working - you can watch the traffic with tcpdump -i tun4 - it’s amazing what a cheap watch is accessing.
Watching the 2 videos in the Readme below helped explain a lot.