NaCl acts as a charge suppressor so that the negatively charged gold nanoparticle and a negatively charged ligand (with thiol) are not longer charge repelled and can experience a chemical bond. NaCl is easily broken into Na+ and Cl- so that the Na+ can pacify the negative charge on the surface of the gold nanoparticle to become less negative and approach zero charge. Other higher charged salts such as Mg2+ impart even greater pacification.
Here are some good references:
Preparation of aptamer-linked gold nanoparticle purple aggregates for colorimetric sensing of analytes
J Liu, Y Lu - Nature protocols, 2006 - nature.com
Homogeneous immunoassay based on aggregation of antibody-functionalized gold nanoparticles coupled with light scattering detection
B Du, Z Li, Y Cheng - Talanta, 2008 - Elsevier
… in 10 mM phosphate-buffered solution (pH 7.0) containing 1.0 mmol l −1 NaCl at room … 7.