Can I put in Github the code of a abandoned plugin I want to fork?

I‘m not a lawyer but a plugin developer.
As far as I know you ca republish any GPL code.
The only thing you should do first is

  • try to reach the author in the forum
  • have a look at the plugin files, maybe there’s an email address where you can try to reach out to the author

If both doesn’t work just fork it.
This is the recommended process to take over a plugin if it seems to be abandoned.