First there's the tried-and-true Gullfoss that's suitable for mixing and most mastering situations. There's Gullfoss Live that features low latency processing for live mixing and tracking. And finally, we have Gullfoss Master that's optimized for mastering.
Until the end of July, 2021, there is a 30% discount. Use code MASTER30 at checkout at soundtheory.com. Current owners of Gullfoss will be able to upgrade at any time for no fee.
There is a two week free-trial period with full functionality.