A search on Acme Maps http://mapper.acme.com/
for Gisselfeld Denmark takes you to a spot two miles south of Haslev, and for Braaby 2 miles southwest of Haslev, so they would be identified with Haslev.
Without at least an approximate age for the siblings, it's a matter of searching Braaby parish records image by image in the online Danish Archives http://www.sa.dk/ao/
The most useful shortcut should be the family's listing in the 1901 Braaby census, since the 1916 census shows them still there.
But since it's not transcribed, that also requires image by image search. I tried "sampling" the 300+ images, but no luck finding the Gisselfeld families so far.