Synthetic peptides derived from GB virus C (GBV-C) have previously been studied in our group for the development of new systems capable of diagnosing diseases caused by this humanotropic virus. We also recently described specific peptide domains of the E2 envelop protein of GBV-C that have the capacity to interfere with the HIV-1 fusion peptide, produce a notable decrease in cellular membrane fusion, and perturb HIV-1 infectivity in a dose-dependent manner. The present work discloses the design and synthesis of both linear and cyclic branched peptides based on a previously reported N-terminal sequence of the GBV-C E2 protein. Immunoassays and cell-cell fusion assays were performed to evaluate their diagnostic value to detect anti-GBV-C antibodies in HIV-1 patients, as well as their putative anti-HIV-1 activity as entry inhibitors. Our results showed that chemical modifications of the selected E2(7-26) linear peptide to afford cyclic architecture do not result in an enhanced inhibition of gp41 HIV-1-mediated cell-cell fusion nor improved sensitivity in the detection of GBV-C antibodies in HIV-1 co-infected patients. Thus, the ELISA data reinforce the potential utility of linear versions of the E2(7-26) region for the development of new peptide-based immunosensor devices for the detection of anti-GBV-C antibodies in HIV-1 co-infected patients.