By switching tags, I was able to determine that GER's RR gun count is down from 3 to 2; I have 3. Was able to locate them all, none have the same name as the one of interest. IDK what 'repair' would look like; if it would be on map like a ship, it's not there. GER has a RR gun under production, but that also has a different name. Does all this mean it was destroyed, or is their some other way to locate a damaged RR gun?
So I still have the same Q: How is a RR gun damaged or destroyed? The wiki doesn't say. The dev diary mentions just one way to damage a RR gun, namely damage to the rail it is sitting on. GER's construction queue shows 1 level of damage to RRs in that state starting 3 days before, but no increased damage after that. (All 3 of my RR guns are in states with RR damage, one of them in the same state.)