Hi,
R u sure its not the moult that is making u think she has lost weight?
Often when shedding they can lose condition so perhaps supplement her diet with some oils "yumega" is meant to be quite good.
Also I oud be cautious re putting weight on a gsd first they are usually pretty sensitive regarding diet change this may cause diarrhoea. 2nd they often have hip problems and excess weight would not do her any favours.
People often see a dog that they think is too skinny when actally they are perfect and we are now so used to seeing even slightly overweight dogs as normal that we percieve the fatter dog as healthy!
You should be able to easily stroke down her side and feel the rib cage (without pressing).
Ps she isnt the one thats had pups is she.....just they normally lose condition after that period.
If shee is way too skinny add cooked potatoes, pasta or extra mixer bikkies to her diet. However do it gradually sudden diet change not so good.