Garde-Schützen-Bataillon is exceedingly rare, and for an infantryman who is also a collector, its the premier infantry unit of the Imperial German Army, - it just doesn't get better... its like the Großdeutchland IR of WWII.
Obviously the rifle is incredibly rare, in any condition, and when brought to my attention by another, - who passed upon it, I have to admit I thought it the coolest rifle that I have seen in a long time. It is incredibly unfortunate that some numbskull vandalized the rifle, placing a ridiculous barrel on it, among other crimes against sanity...
To aid your project I will attach pictures of the only other GS related rifle I have encountered, a stock I once owned years ago. Interestingly I picked it up through Lewis (S/42) who use to have a rather decent little Imperial collection, he lives in Australia and there is some speculation that it may have been brought back to Australia by a soldier. The G.S served the last stages of the war in the Balkans, so perhaps it encountered some Australian units, which would explain the stocks presence in AUS and the rarity of surviving items from the btl. (though being such a small unit, such material would be scarce regardless..)
Notice the different maker on this stock vs. your example, 7661 being Danzig in my opinion, though this is more a matter of elimination than evidence. Factory rifles from 1899-1900 are almost unheard of, handfuls across the spectrum, and none are problem free. Mostly trends revolve around components... Anyway, having another example to gauge your project by should help. (7661 did not have the early BL, so a latter upgrade)