I'd imagine that it might be slightly uncomfortable at first, but not painful. I think that's typically why they want the dog to wear the collar for a few weeks without actually using it - for them to get used to something so tight on their neck.
You might need longer contact points, though, since GSDs have the double coat. I've seen people rub the receiver up and down on the dogs neck to push away fur and get better contact with the skin.
+ Kalbi CGC 6/21/2015
+ Blackthorn's Out of the Frying Pan NW1 SCN BCAT SIN RATN TKN (Katsu) 9/12/2017
+ Blackthorn's Skofnung RATI RATN RATO (Steel) 07/23/2018