I apologize for the inconvenience. I did made another video using a similar GA-400 series model but in different color, GA-400LY lime accent series. Link here: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-WDlj4ksAbx0.html
GD-100 model has quick access to 4 worldtime complication and other functions are just basic. GA-400 model has quick access to countdown timer or features regarding to timing things and other basic functions as well. The thing about GD-100 is that it is fully digital and the backlight is bright which going to be easy to use at night. A GA-400 series are analog digital which is also great at telling time but the single orange backlight not going to help much with other complications at night; only the analog hand could be seen and not digital display. But if you just want to tell the time, a GA-100 good enough. If you want more, then go for GA-400. Full functions look on GA-400 here :ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Wot04qA16Ac.html
I made like another 4 videos regarding a GA-400 series because of this. Go check them out. I didn't make any commentary or anything at all in my videos before year 2016. I apologize for the inconvenience.
The specialty of a GA-400 is that you'll have quick access to countdown timer function at home time. The small dial is for that purposes. 1 minute - 60 minutes indicator. I already shows how it work in this video but if you want to learn more about this watch. I already created a whole tutorial video dedicated for this model. Link here :ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Wot04qA16Ac.html