Will a sandwich assay work with a GST 26 antibody/protein?
This is completely dependent on the type of antibody you are using and the epitope specificity of that antibody (or antibodies). Because you haven't given any more detail, it's very hard to give you a definitive answer.

GST is known to dimerise, so even a monoclonal antibody might work in your sandwich assay, as long as the (identical) epitopes are far enough apart in the dimeric structure to allow two antibodies to bind.

If you're dealing with a polyclonal antibody mixture, then your chances are better. But even still, you won't know until you try it.