Why Germany? The Gotthard Basis Tunnel is in Switzerland and worker pay in Switzerland is considerably higher in Switzerland than in Germany and also probably the US
while i'm sure the issue with the costs here and there isn't labor/salary, googled out of curiosity - for welder in US it is $30-50K/year avg, in Germany - the government mandated minimum in non-East Germany is something like 14euro/hour, in the East - 11 euro/hour.
I also would be very surprised if the Germany/Switzerland were "outsourcing" the job to Chinese companies like the US does with big infrastructure projects. I mean everybody who ever dealt with outsourcing up close knows what in reality it is completely opposite to "cost savings", instead it is a way to "digest" even more money than it would be otherwise.
You have to remember that tunnel jobs are almost always run by a municipality which means that they're almost always union. I would imagine it is similar in Switzerland.
Tunneling is a niche industry and commands a high price.