Dear Mustafa,

Please try Avogadro, It is very easier to generate initial structures

and save them in xyz format.

Also you can optimize your cluster with molecular mechanic methods implemented in Avogadro, before DFT (or other quantum chemistry) calculations.

Bests

Dear Mustafa,

Best answer to you question is already given by Mr. Yavar. Avogadro should be your software of choice for present problem but still if you want to strictly use gaussview, try to utilize "clean" function of gaussview ,which create good guess from a initial structure without any optimization using some predetermined parameters.My suggestion is draw part of the cluster,clean it then go for the next part clean the whole and keep continuing this way until the whole structure is completed.