Right now the vast majority of it is open source. There is a component that is not: the rails server, but that's pretty tiny. The client can be developed on without that. If there's some interest I'm happy to make what's existing available.
I can't make guarantees about the distant future. There's a ton of work I would love to see happen with Guesstimate, and my guess is that much of it would only be possible if it becomes a company. This can still mean that it can be mostly open source, but I really have little idea what the situation would be at that time.