Is IgG a secondary antibody?

IgG is not a secondary antibody. Anti-IgG is secondary antibody.

A primary antibody is generally IgG isotype. You will need an anti-IgG H&L (Heavy & Light chains) antibody as a secondary antibody, which reacts with both the heavy and light chains of the primary IgG antibody. In addition, this anti-IgG H&L antibody also reacts with other immunoglobulin classes such as IgM and IgA of the species of the primary antibody, because all immunoglobulins in a given species have the same light chains.