Gems are not shares of the company. But if the company is successful they would surely be worth a good bit because of the demand.
Gems can represent multiple things, some like to use it as shares of the application, some use it as an in-app currency. It's pretty versatile where you can shape it for what purpose you want, but eventually they will be used more as the in-app currency to drive the whole economical network within the app.
My take, Gems are a private token withing the GemGems eco system/economy. The value of a Gem is based on the value of the network, ie, active users.
Gems are not a security that gives you a fractional ownership of the GetGems "entity". Even though GetGems was financed by selling their private token for a fungible currency (Bitcoin) you didn't buy ownership.
So, if GetGems is sold to Google for $1B your share would be 0.
That is, unless GetGems issues a security (stock/counterpary asset) and your purchased those.