I suggest reading the manual about airplane configuration, GSX comes with a selection of default and add-on airplane, but it cannot possibly include all of them, that's why there's a fully feature configuration editor.
If you want to use an unsupported airplane, it must be configured before use. This, assuming the airplane has a different name/folder than the original 787, which seems to indicate why is not recognized. The manual explains how to know if an airplane has configuration or not at Page 49.