Free air or waveguide wavelength?
You use the guide wavelength. This is because the wavelength in the waveguide is different than the wavelength in free space. When a wave travels through the waveguide its wavelength changes. You need to solve Maxwell’s equations with the appropriate boundary conditions in order to determine what the wavelength will be in a specific waveguide.

The frequency remains the same. Saying “lambda g frequency” does not make much sense, because lambda g is the wavelength. You use the wavelength not the frequency.