guided wavelength and effective aperture

which is required for estimating effective aperture of an antenna lambda

or lambda g

g (guided wavelength)?

well, is your entire antenna inside of a waveguide?

If you use a horn antenna with a waveguide feeder, then in the waveguide the lambda is the guide wavelength and outside horn aperture the lambda is in the air. The horn or another antenna transforms the guided wave into the free-space wave.
Antenna aperture is the geometric aperture of its mouth, rectangular or circular, and it determines appropriate beam width in E ad H plane or in any chosen plane.

i just want to know whether lambda or lambda g is used in the eqn D=4

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2 *Ae (for the case of dielectric loaded antenna)