The big advantage of GravityLight is that you can pull the weight once and then you have electricity for a relatively long period (I read somewhere it's 30 minutes). This is ideal when you need to have both hands free to do some work. This long period is also a must if you want it to replace the kerosene lamps that are currently used in rural Africa. The latter has always been the main goal of the inventors of the GravityLight. Replacing kerosene lamps by GravityLights could:
- save billions of liters of kerosene (and thus reduce CO2 emissions),
- save its users a lot of money (many African families spend around 20% of their income on kerosene),
- avoid fires and severe burning wounds (kerosene lamps are sometimes knocked over which can have disastrous consequences),
- reduce exposure to toxic fumes.
Other mechanically-powered lights only provide light for a very short period or require you to generate electricity continuously, so they aren't very suitable as a replacement.
AFAIK the GravityLight is not for sale in any shops. The only way to get one is if you contributed to the crowd-funding campaigns. A while back there was second crowd-funding round for an improved GravityLight 2.0. If you donated $70 USD or more you'll receive one of the new lights by end 2016. Sadly the website now says that the campaign is closed, so it may not be possible to ‘buy’ it anymore.
I am not aware of any similar lights that also use gravity, at least not working ones. In 2008 someone did come up with a similar idea called the Gravia floor lamp, but that wasn't developed into a prototype.
There seem to be a few patents on the GravityLight design (e.g. this one and this one), so that could be the reason for the lack of similar lamps.