Correlation between structure, electronic properties, and magnetism in $Co_{x}Gd_{1-x}$ thin amorphous films was investigated. The thickness averaged properties of covered thin films are consistent with those of in-depth homogeneous amorphous alloys. In spite of that, x-ray magnetic circular dichroism (XMCD) and photoemission measurements on as-grown films have shown signatures consistent with a lateral gradient composition and a Gd surfactant effect. A tendency of Gd atoms to migrate to the surface followed by an oxidation down to 8 Å has been evidenced. Our results further demonstrate the extreme sensitivity of the magnetic properties of ferrimagnetic alloys to their structure and local concentration.