For any antibofy assay, you generally create both a reference standard and positive controls by collecting serum from animals that have been vaccinated with the target antigen. So in your case, you will need to vaccinate some non-human primates with GFP, then collect serum. If you want to use actual units of concentration, you will also need to establish the concentration of the reference standard using independent methods. For an example of how this can be done in a human example, you can look at Semenova et al. Clinical and Diagnostic Laboratory Immunology, Sept. 2004, p. 919–923, DOI: 10.1128/CDLI.11.5.919–923.2004.

If you don't want to raise your own anti-sera, you may be able to use some rabbit anti-GFP serum that is commercially available as a control, as long as your secondary antibody isn't species-specific. A protein-G conjugate could work for that.

Many thanks for your kind answer. Actually I' m trying to find a secondary antibody able to cross react both with primate IgG and with a rabbit anti-GFP IgG....