How common was for the mechanics to sit out in the engine gondola? Was it just the Germans that used this method? Must have been lonely.....!

The Germans did have larger gondolas on their large Zeppelin Staaken bombers. This was a pusher / tractor arrangement where two engines were in the same gondola. That allowed for the mechanic to travel in the gondolas. It wasn't a practical measure for aircraft with smaller wingspans.

Even the Handley Page 0/400 did not allow for the mechanic to travel inside its gondola. The post war 0/1500 may have been redesigned to allow for this. (I'll have to do some checking.)

The large Italian Caproni bomber may have had gondolas on some variants that allowed for the mechanic to travel in the gondolas. (I'll have to do some checking.)

Thank you O' Enlighted One!

WWI saw also the development of electrically heated socks and flight suits.

Lonely? Perhaps. Cramped? Quite likely. Surely they must have kept their tootsies warm?