This Letter describes a double-sided process to fabricate freestanding membrane devices on a GaN-on-silicon platform. The photoluminescence measurement is taken to characterize the optical performance. A large portion of the excited light from InGaN/GaN multiple quantum wells is trapped as waveguide modes and propagates in different directions. Experimental results show that the propagation direction of the waveguide mode can be converted into the direction normal to the surface at the edge of a freestanding membrane, and the emitted light is attenuated due to light propagation loss before it gets out from the edge. Subwavelength grating can also convert waveguide modes into air modes on a freestanding membrane. These results suggest that the emission efficiency can be greatly improved by employing more efficient light extraction methods and that GaN-based photonic waveguides are promising in the visible range.