Currently the only way to receive the CS:GO SDK is to have a paid version of Prime Status on your account. This means you must have Prime Status on your account through any of the following methods:
By purchasing Prime Status through the Steam Store.
By adding a retail CD key for CS:GO to your Steam account.
Users who receive Prime Status through leveling their CS:GO account to Profile Level 21 are not granted the SDK. To receive the SDK you will need to purchase Prime Status through the Steam Store or add a retail CD key for the game to your account.
If you'd like to provide feedback on this decision please do so through CS:GO's Steam Discussions - the CS:GO developers regularly check Steam Discussions for feedback from the Community.