Can i use part of the php code of another author?

All WordPress themes that sell on Themeforest, have php code is GPL license or not?

I want to use in my WordPress theme a piece of php code from the theme, which another author sells on Themeforest
I also want to sell my theme on Themeforest, where i use a part of the php code of another author.

  1. Do i have to get permission from the author of the theme or not, if the php code with GPL license?
  2. Do i have to specify in this copyright, link to the author?
  3. Themeforest allow to sell this my theme?