Using the aperture setting will not degrade the image quality to the extent that adjusting the gain will. While darkening might in some ways differ, it is especially true that using gain to make the sensor(s) MORE sensitive to light will introduce more graininess in your video.
Using aperture or shutter speeds to manually control the light quality will avoid the general image degradation that gain usually introduces to a greater or lesser degree.
This is a general comment and there are a wide range of specifics regarding the use and purpose of either approach. Just be aware that gain (boosting the signal electronically “artificially”) will usually introduce “noise” more so than working with aperture/shutter speeds.
Your aperture setting WILL however have an effect on your depth of focus wide open = shallow; closed all the way = gives a deeper range of image that remains in focus, “infinity” more or less, to use the photographic term.
.
