The formation of gold nanoparticle (Au NP)-loaded γ-polyglutamic acid (γ-PGA) nanogels (NGs) for computed tomography (CT) imaging of tumors is reported. γ-PGA with carboxyl groups activated by 1-ethyl-3-[3-(dimethylamino)propyl] carbodiimide hydrochloride is first emulsified to form NGs and then in situ chemically cross-linked with polyethylenimine (PEI)-entrapped Au NPs with partial polyethylene glycol (PEG) modification ([(Au0)200-PEI·NH2-mPEG]). The formed γ-PGA-[(Au0)200-PEI·NH2-mPEG] NGs with a size of 108.6 ± 19.1 nm display an X-ray attenuation property better than commercial iodinated small-molecular-contrast agents and can be uptaken by cancer cells more significantly than γ-PGA-stabilized single Au NPs at the same Au concentrations. These properties render the formed NGs with an ability to be used as an effective contrast agent for the CT imaging of cancer cells in vitro and a tumor model in vivo. The developed hybrid NGs may be promising for the CT imaging or theranostics of different biosystems.