I'm German, so I can tell you that there's no such thing as a "gigger-gagger" as far as I can tell (I'm from the north west of Germany, so it may be possible the word is more common in the south/Bayern at the Oktoberfest or something like that). I searched the web for it and also asked my family, they've never heard about it. Telling from the sound, I would also suggest that it could be something like a "wake-up" signal - like the word we use for mocking a rooster's sound in the morning: "kikeriki" ("ki" is spoken like the word "key", so it's 'key-ke-re-keeeey').
I think the word "gigger-gagger" could also describe the fact that the person is quite known for being strict and to make people be silent in his 'hotel', so he's always shouting and blustering without his guests really listening. So because he's always complaining or something, his words become a "gigger-gagger" - a never ending complaint noone really takes serious, like a "bla bla bla". In this case, the word is also a sound or signal, but not to wake someone up but to make someone shut up or behave.

Maybe this is helpful, too:
http://www.mudcat.org/thread.cfm?threadid=34802
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