How can you add a gift premium to pledge
For our projects we always use Contributions (instead of pledges); in a nutshell we use: Contributions  - with Status Pending - and with a receive date in the future.

Added: when you receive a check for eg 3 of such pending Contributions you can hit Edit -> record the chq payment, update the receive date to the actual one (the one you had was just the 'promised by' one and toggle their contribution status to 'completed'

You can use any of the Contribution reports to find the Pending ones - you can use all Searches e.g. Final all that are Pending this week (may need follow up reminder) - and you can use Premiums.