Can the GEMINI select a specific spectral band?

No, the GEMINI does not act as a monochromator, so it is not possible to select a single wavelength at the output, given a broad spectrum in the input.

The GEMINI Interferometer always transmits the entire spectrum of the incoming light, introducing a spectral modulation with a period that depends on the delay of the interferometer. For more information on the working principle of the device, please refer to this video.