After you receive your Gullfoss serial number (activation key) from SudeepAudio.com:

Simply download the software and install it by opening the archive and then double-clicking the installer. A dialog will appear that guides you through the rest of the installation. Make sure you have the latest iLok License Manager installed, then activate your product from within the iLok License Manager.

In case you’re still using the legacy Soundtheory copy protection:

– Upon first instantiation of the plug-ins you will be presented with an info page where you can hit the ‘Authorize’ button.
– After entering your account credentials, you will be able to use your product.

You can also convert your license to iLok by going to your account page and click on the ‘Convert to iLok’ button.

Follow the step-by-step installation instructions given here