Gems is deceptively named but it's essentially just for posting interesting things that gets discussions or Diggs, or being early to post something. It has nothing to do with crypto etc.
Source: I've been using the app since the alpha started.
Source: I've been using the app since the alpha started.
They didn't really "sell usernames", unless you also call buying a paid app with social features "buying a username".
And as far as "Gems", looks like bragging rights. This is what clicking (?) currently says:
> Gems are earned by being amongst the very first to Digg a post that trends across the platform. The earlier you are to discovering and Digging the post, the more Gems you’ll earn.