As your Minions complete tasks, Gru tokens will become available to you for doing so. Make sure you don’t forget to claim them. Minion Rush is sneaky about not really explaining this.

In order to claim tokens for the achievements you’ve completed, tap on the Goals menu from the main screen. Along the top you’ll need to toggle from the goals tab to the achievements tab. Scroll through the list and see if any are completed. If they are you’ll see a claim button next to it.