Dear Marie,
First, welcome to the ABM and Gama communities.
Regarding your concern, I'm afraid you won't find any magical migration algorithm to translate any netlogo code into gaml one. However, Gama has plenty of exemple model implementations in the native library and many Schelling models are included (if I remember correctly under the segregation folder of toy models). You can compare implementations and begin to understand how concepts of the model are translated into gaml. Doing that you can come back here if you have any trouble.
Best use of this mailing list is to provide concret issues about implemented model, with code snippet.
Best,